Nucleotide sequences encoding maize RAD51

ABSTRACT

Nucleic acid sequences encoding two RAD51 recombinases active in maize plants are provided. cDNA sequences including the ZmRAD51 coding sequences and unique 3′-untranslated regions which are useful as RFLP probes, are also provided. The production of plasmids containing a nucleic acid sequence encoding a ZmRAD51 fusion protein, as well as the use of the plasmids to introduce the ZmRAD51 coding sequence into a host cell, such as maize cell, are also disclosed.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/074,745, filed Feb. 13, 1998 and is herein incorporated by reference.

BACKGROUND OF THE INVENTION

Transgenic plant product development by conventional transformation andbreeding efforts is a slow and unpredictable process. Gene targetingsystems can overcome problems with expression variability, unpredictableimpacts of random gene insertion on agronomic performance, and the largenumber of experiments that need to be conducted. Such systems can alsoprovide approaches to manipulating endogeneous genes. Of course,targeting system requires the ability to focus the recombination processto favor recovery of desired targeting events.

The natural cellular DNA repair and recombination machinery consists ofa complex array of protein components interacting in a highly controlledmanner to ensure that the fidelity of the genome is conserved throughoutthe many internal events or external stimuli experienced during eachcell cycle. The ability to manipulate this machinery requires anunderstanding of how specific proteins are involved in the process, andhow the genes that encode those proteins are regulated. Since theprimary approaches to gene targeting involve recombinases, whetheroperating in their natural in vivo environment (as during normalrecombination) or as part of schemes that involve pretreatment ofsubstrates so as to associate DNA with a recombinase and increaseefficiency of targeting (e.g., double D-loop), there is a continuingneed to isolate and characterize the genes for these molecules. Becausemany different protein components may be involved in gene targeting, theavailability of host-specific genes and proteins could avoid possibleproblems of incompatibility associated with molecular interactions dueto heterologous components.

Sequences for the bacterial RecA recombinase and functional homologsfrom yeast and several animal species have been disclosed in variouspublicly accessible sequence databases. Numerous publicationscharacterizing these recombinases exist (see, e.g., Kowalczykowski etal., Annu. Rev. Biochem., 63:991-1043 (1994)). Reports of the use ofbacterial RecA in association with DNA sequences to manipulatehomologous target DNA, including improvement of the efficiency of genetargeting in non-plant systems, have been published (see, e.g., PCTpublished Patent Application Nos. WO 87/01730 and WO 93/22443).

The catalysis of in vitro pairing and strand exchange between circularviral single strand DNA (“ss DNA”) and linear duplex DNA (“ds DNA”) by aRAD51 recombinase from S. cerevisiae has also been reported (see, e.g.,Sung, Science, 265:1241-43 (1994); Kanaar, et al., Nature 391:335-338(1998); Benson, et al. Nature 391:401-410 (1998)). To date, work withrecombinase enzymes in plants, however, has been very limited.Accordingly, there is an ongoing need for the identification andcharacterization of the functional activities of recombinase enzymeswhich may offer improved and expanded methods for use in plant systems,particularly agriculturally important crop species such as maize.

SUMMARY OF THE INVENTION

Polynucleotide sequences, which encode putatively active RAD51recombinases, have been isolated from maize. Specifically, cDNA clonesZmRAD51A (SEQ ID NOS: 1) and ZmRAD51B (SEQ ID NOS: 5) from a maizetassel library have been identified and sequenced. The cDNA sequencesinclude 3′-untranslated regions (SEQ ID NOS: 4 and 8) suitable for usein making gene-specific probes, e.g., which can be used to map the locusof the respective ZmRAD51 gene in an RFLP map of a maize population. TheRFLP probes are typically at least 15 nucleotide residues, althoughsmaller and larger sizes may also be used. The present invention alsoincludes expression cassettes, vectors, and host cells that incorporatethe ZmRAD51 genes. Monocot cells, such as maize cells, are particularlypreferred as host cells. In addition, a nuclear localization sequencecomprising the 5′ end of the ZmRAD51 gene is identified.

In a further aspect, the present invention relates to an isolatedprotein comprising a polypeptide of at least 10 contiguous amino acidsencoded by the isolated nucleic acid of ZmRAD51A or ZmRAD51B. In someembodiments, the polypeptide has a sequence selected from the groupconsisting of SEQ ID NOS: 3 and 7.

In yet another aspect, the present invention relates to a transgenicplant comprising a expression cassette comprising a plant promoteroperably linked to any of the isolated nucleic acids of the presentinvention. Methods for modulating, in a transgenic plant, the expressionof the nucleic acids of the present invention are also included. In someembodiments, the transgenic plant is Zea mays. The present inventionalso provides transgenic seed from the transgenic plant.

In a further aspect, the present invention relates to a method of makingmaize recombinase by transforming or transfecting a host cell with anexpression vector containing one of the isolated nucleic acids of thepresent invention and purifying the recombinase protein from the hostcell. In some embodiments, the host cell is a bacterial cell, a yeastcell, or a plant cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a map of a plasmid designated PHP8060 derived from theinsertion of a modified ZmRAD51A gene between a maize ubiquitin promoterand a potato proteinase inhibitor (“PinII”) terminator in a pUC19plasmid backbone.

FIG. 2 shows a map of a plasmid designated PHP8103 derived from theinsertion of a modified ZmRAD51B gene between a maize ubiquitin promoterand a potato proteinase inhibitor (“PinII”) terminator in a pUC19plasmid backbone.

FIG. 3 shows a map of a plasmid designated PHP8744 derived from theinsertion of a GFPm gene 5′ to the start of the modified ZmRAD51A genein PHP8060 to create a sequence encoding a GFP/ZmRAD51A fusion protein.Optionally, the modified ZmRAD51B gene could be placed instead of themodified ZmRAD51A gene, to form a GFP/ZmRAD51B fusion gene.

DETAILED DESCRIPTION OF THE INVENTION

Full-length cDNA clones for two maize homologs of the yeast RAD51 genehave been isolated. Significant transcription levels have been detectedprimarily in immature ears and anthers that contain cells progressingthrough the early stages of meiosis. The two isolated cDNAs, however,are more closely related to RAD51 family members expressed in mitoticcells than to the meiosis-specific homologs from plants (LIM15) andyeast (DMC1). RFLP mapping indicates that the Zea mays genome containstwo genes encoding different variants of the ZmRAD51 recombinase enzyme.The genes encoding each protein (ZmRAD51A and ZmRAD51B) are unlinked,and their map positions do not correspond to any known maize mutationswith a meiotic phenotype. In addition to providing nucleotide sequences,which can be used to produce substantially purified RAD51 proteins, theresults presented herein indicate that sequences from the ZmRAD51A andZmRAD51B cDNA clones can serve both as sources of hybridization probesfor RAD51-related genes, as well as novel and unique RFLP probes forapplications such as mapping or marker-assisted selection in maize.

The isolated polynucleotides and polypeptides of the present inventioncan be used over a broad range of plant types, particularly monocotssuch as the species of the family Gramineae including Hordeum, Secale,Triticum, Sorghum (e.g., S. bicolor) and Zea (e.g., Z. mays). Theisolated nucleic acid and proteins of the present invention can also beused in species from the genera: Cucurbita, Rosa, Vitis, Juglans,Fragaria, Lotus, Medicago, Onobrychis, Trifolium, Trigonella, Vigna,Citrus, Linum, Geranium, Manihot, Daucus, Arabidopsis, Brassica,Raphanus, Sinapis, Atropa, Capsicum, Datura, Hyoscyamus, Lycopersicon,Nicotiana, Solanum, Petunia, Digitalis, Majorana, Ciahorium, Helianthus,Lactuca, Bromus, Asparagus, Antirrhinum, Heterocallis, Nemesis,Pelargonium, Panieum, Pennisetum, Ranunculus, Senecio, Salpiglossis,Cucumis, Browaalia, Glycine, Pisum, Phaseolus, Lolium, Oryza, and Avena.

Nucleotide Sequence Encoding ZmRAD51A & ZmRAD51B Proteins

As used herein, “nucleic acid” includes reference to adeoxyribonucleotide or ribonucleotide polymer in either single- ordouble-stranded form, and unless otherwise limited, encompasses knownanalogues having the essential nature of natural nucleotides in thatthey hybridize to single-stranded nucleic acids in a manner similar tonaturally occurring nucleotides (e.g., peptide nucleic acids).

By “nucleic acid library” is meant a collection of isolated DNA or RNAmolecules which comprise and substantially represent the entiretranscribed fraction of a genome of a specified organism. Constructionof exemplary nucleic acid libraries, such as genomic and cDNA libraries,is taught in standard molecular biology references such as Berger andKimmel, Guide to Molecular Cloning Techniques, Methods in Enzymology,Vol. 152, Academic Press, Inc., San Diego, Calif. (Berger); Sambrook etal., Molecular Cloning—A Laboratory Manual, 2nd ed., Vol. 1-3 (1989);and Current Protocols in Molecular Biology, F. M. Ausubel et al., Eds.,Current Protocols, a joint venture between Greene Publishing Associates,Inc. and John Wiley & Sons, Inc. (1994).

The terms “isolated” refers to material, such as nucleic acid orprotein, which is: (1) substantially or essentially free from componentsthat normally accompany or interact with it as found in its naturallyoccurring environment. The isolated material optionally comprisesmaterial not found with the material in its natural environment; or (2)if the material is in its natural environment, the material has beensynthetically (non-naturally) altered by deliberate human interventionto a composition and/or placed at a location in the cell (e.g., genomeor subcellular organelle) not native to a material found in thatenvironment. The alteration to yield the synthetic material can beperformed on the material within or removed from its natural state. Forexample, a naturally occurring nucleic acid becomes an isolated nucleicacid if it is altered, or if it is transcribed from DNA which has beenaltered, by means of human intervention performed within the cell fromwhich it originates. See, e.g., Compound and Methods for Site DirectedMutagenesis in Eukaryotic Cells, Kmiec, U.S. Pat. No. 5,565,350; In VivoHomologous Sequence Targeting in Eukaryotic Cells; Zarling et al.,PCT/US93/03868. Likewise, a naturally occurring nucleic acid (e.g., apromoter) becomes isolated if it is introduced by non-naturallyoccurring means to a locus of the genome not native to that nucleicacid. Nucleic acids, which are “isolated” as defined herein, are alsoreferred to as “heterologous” nucleic acids.

As used herein “operably linked” includes reference to a functionallinkage between a promoter and a second sequence, wherein the promotersequence initiates and mediates transcription of the DNA sequencecorresponding to the second sequence. Generally, operably linked meansthat the nucleic acid sequences being linked are contiguous and, wherenecessary to join two protein coding regions, contiguous and in the samereading frame.

As used herein, “polynucleotide” includes reference to adeoxyribopolynucleotide, ribopolynucleotide, or analogs thereof thathave the essential nature of a natural ribonucleotide in that theyhybridize, under stringent hybridization conditions, to substantiallythe same nucleotide sequence as naturally occurring nucleotides and/orallow translation into the same amino acid(s) as the naturally occurringnucleotide(s). A polynucleotide can be full-length or a subsequence of anative or heterologous structural or regulatory gene. Unless otherwiseindicated, the term includes reference to the specified sequence as wellas the complementary sequence thereof. Thus, DNAs or RNAs with backbonesmodified for stability or for other reasons are “polynucleotides” asthat term is intended herein. Moreover, DNAs or RNAs comprising unusualbases, such as inosine, or modified bases, such as tritylated bases, toname just two examples, are polynucleotides as the term is used herein.It will be appreciated that a great variety of modifications have beenmade to DNA and RNA that serve many useful purposes known to those ofskill in the art. The term polynucleotide as it is employed hereinembraces such chemically, enzymatically or metabolically modified formsof polynucleotides, as well as the chemical forms of DNA and RNAcharacteristic of viruses and cells, including among other things,simple and complex cells.

The terms “polypeptide”, “peptide” and “protein” are usedinterchangeably herein to refer to a polymer of amino acid residues. Theterms apply to amino acid polymers in which one or more amino acidresidue is an artificial chemical analogue of a corresponding naturallyoccurring amino acid, as well as to naturally occurring amino acidpolymers. The essential nature of such analogues of naturally occurringamino acids is that, when incorporated into a protein, that protein isspecifically reactive to antibodies elicited to the same protein butconsisting entirely of naturally occurring amino acids. The terms“polypeptide”, “peptide” and “protein” are also inclusive ofmodifications including, but not limited to, glycosylation, lipidattachment, sulfation, gamma-carboxylation of glutamic acid residues,hydroxylation and ADP-ribosylation. It will be appreciated, as is wellknown and as noted above, that polypeptides are not always entirelylinear. For instance, polypeptides may be branched as a result ofubiquitination, and they may be circular, with or without branching,generally as a result of posttranslation events, including naturalprocessing event and events brought about by human manipulation which donot occur naturally. Circular, branched and branched circularpolypeptides may be synthesized by non-translation natural process andby entirely synthetic methods, as well. Further, this inventioncontemplates the use of both the methionine-containing and themethionine-less amino terminal variants of the protein of the invention.

The following terms are used to describe the sequence relationshipsbetween two or more nucleic acids or polynucleotides: (a) “referencesequence”, (b) “comparison window”, (c) “sequence identity”, (d)“percentage of sequence identity”, and (e) “substantial identity”.

(a) As used herein, “reference sequence” is a defined sequence used as abasis for sequence comparison. A reference sequence may be a subset orthe entirety of a specified sequence; for example, as a segment of afull-length cDNA or gene sequence, or the complete cDNA or genesequence.

(b) As used herein, “comparison window” includes reference to acontiguous and specified segment of a polynucleotide sequence, whereinthe polynucleotide sequence may be compared to a reference sequence andwherein the portion of the polynucleotide sequence in the comparisonwindow may comprise additions or deletions (i.e., gaps) compared to thereference sequence (which does not comprise additions or deletions) foroptimal alignment of the two sequences. Generally, the comparison windowis at least 20 contiguous nucleotides in length, and optionally can be30, 40, 50, 100, or longer. Those of skill in the art understand that toavoid a high similarity to a reference sequence due to inclusion of gapsin the polynucleotide sequence a gap penalty is typically introduced andis subtracted from the number of matches.

Methods of alignment of sequences for comparison are well-known in theart. Optimal alignment of sequences for comparison may be conducted bythe local homology algorithm of Smith and Waterman, Adv. Appl. Math. 2:482 (1981); by the homology alignment algorithm of Needleman and Wunsch,J. Mol. Biol. 48: 443 (1970); by the search for similarity method ofPearson and Lipman, Proc. Natl. Acad. Sci. 85: 2444 (1988); bycomputerized implementations of these algorithms, including, but notlimited to: CLUSTAL in the PC/Gene program by Intelligenetics, MountainView, Calif., GAP, BESTFIT, BLAST,, FASTA, and TFASTA in the WisconsinGenetics Software Package, Genetics Computer Group (GCG), 575 ScienceDr., Madison, Wis., USA; the CLUSTAL program is well described byHiggins and Sharp, Gene 73: 237-244 (1988); Higgins and Sharp, CABIOS 5:151-153 (1989); Corpet, et al., Nucleic Acids Research 16: 10881-90(1988); Huang, et al., Computer Applications in the Biosciences 8:155-65 (1992), and Pearson, et al., Methods in Molecular Biology 24:307-331 (1994); preferred computer alignment methods also include theBLASTP, BLASTN, and BLASTX algorithms. Altschul, et al., J. Mol. Biol.215: 403-410 (1990). Alignment is also often performed by inspection andmanual alignment.

Unless otherwise stated, sequence identity/similarity values providedherein refer to the value obtained using the BLAST 2.0 suite of programsusing default parameters. Altschul et al., Nucleic Acids Res.25:3389-3402 (1997). Software for performing BLAST analyses is publiclyavailable, e.g., through the National Center for BiotechnologyInformation. This algorithm involves first identifying high scoringsequence pairs (HSPs) by identifying short words of length W in thequery sequence, which either match or satisfy some positive-valuedthreshold score T when aligned with a word of the same length in adatabase sequence. T is referred to as the neighborhood word scorethreshold (Altschul et al., supra). These initial neighborhood word hitsact as seeds for initiating searches to find longer HSPs containingthem. The word hits are then extended in both directions along eachsequence for as far as the cumulative alignment score can be increased.Cumulative scores are calculated using, for nucleotide sequences, theparameters M (reward score for a pair of matching residues; always >0)and N (penalty score for mismatching residues; always <0). For aminoacid sequences, a scoring matrix is used to calculate the cumulativescore. Extension of the word hits in each direction are halted when: thecumulative alignment score falls off by the quantity X from its maximumachieved value; the cumulative score goes to zero or below, due to theaccumulation of one or more negative-scoring residue alignments; or theend of either sequence is reached. The BLAST algorithm parameters W, T,and X determine the sensitivity and speed of the alignment. The BLASTNprogram (for nucleotide sequences) uses as defaults a wordlength (W) of11, an expectation (E) of 10, a cutoff of 100, M=5, N=−4, and acomparison of both strands. For amino acid sequences, the BLASTP programuses as defaults a wordlength (W) of 3, an expectation (E) of 10, andthe BLOSUM62 scoring matrix (see Henikoff & Henikoff (1989) Proc. Natl.Acad. Sci. USA 89:10915).

In addition to calculating percent sequence identity, the BLASTalgorithm also performs a statistical analysis of the similarity betweentwo sequences (see, e.g., Karlin & Altschul, Proc. Nat'l. Acad. Sci. USA90:5873-5787 (1993)). One measure of similarity provided by the BLASTalgorithm is the smallest sum probability (P(N)), which provides anindication of the probability by which a match between two nucleotide oramino acid sequences would occur by chance.

BLAST searches assume that proteins can be modeled as random sequences.However, many real proteins comprise regions of nonrandom sequences,which may be homopolymeric tracts, short-period repeats, or regionsenriched in one or more amino acids. Such low-complexity regions may bealigned between unrelated proteins even though other regions of theprotein are entirely dissimilar. A number of low-complexity filterprograms can be employed to reduce such low-complexity alignments. Forexample, the SEG (Wooten and Federhen, Comput. Chem., 17:149-163 (1993))and XNU (Claverie and States, Comput. Chem., 17:191-201 (1993))low-complexity filters can be employed alone or in combination.

(c) As used herein, “sequence identity” or “identity” in the context oftwo nucleic acid or polypeptide sequences includes reference to theresidues in the two sequences which are the same when aligned formaximum correspondence over a specified comparison window. Whenpercentage of sequence identity is used in reference to proteins it isrecognized that residue positions which are not identical often differby conservative amino acid substitutions, where amino acid residues aresubstituted for other amino acid residues with similar chemicalproperties (e.g. charge or hydrophobicity) and therefore do not changethe functional properties of the molecule. Where sequences differ inconservative substitutions, the percent sequence identity may beadjusted upwards to correct for the conservative nature of thesubstitution. Sequences which differ by such conservative substitutionsare said to have “sequence similarity” or “similarity”. Means for makingthis adjustment are well-known to those of skill in the art. Typicallythis involves scoring a conservative substitution as a partial ratherthan a full mismatch, thereby increasing the percentage sequenceidentity. Thus, for example, where an identical amino acid is given ascore of 1 and a non-conservative substitution is given a score of zero,a conservative substitution is given a score between zero and 1. Thescoring of conservative substitutions is calculated, e.g., according tothe algorithm of Meyers and Miller, Computer Applic. Biol. Sci., 4:11-17 (1988) e.g., as implemented in the program PC/GENE(Intelligenetics, Mountain View, Calif., USA).

(d) As used herein, “percentage of sequence identity” means the valuedetermined by comparing two optimally aligned sequences over acomparison window, wherein the portion of the polynucleotide sequence inthe comparison window may comprise additions or deletions (i.e., gaps)as compared to the reference sequence (which does not comprise additionsor deletions) for optimal alignment of the two sequences. The percentageis calculated by determining the number of positions at which theidentical nucleic acid base or amino acid residue occurs in bothsequences to yield the number of matched positions, dividing the numberof matched positions by the total number of positions in the window ofcomparison and multiplying the result by 100 to yield the percentage ofsequence identity.

(e) (i) The term “substantial identity” of polynucleotide sequencesmeans that a polynucleotide comprises a sequence that has at least 70%sequence identity, preferably at least 80%, more preferably at least 90%and most preferably at least 95%, compared to a reference sequence usingone of the alignment programs described using standard parameters. Oneof skill will recognize that these values can be appropriately adjustedto determine corresponding identity of proteins encoded by twonucleotide sequences by taking into account codon degeneracy, amino acidsimilarity, reading frame positioning and the like. Substantial identityof amino acid sequences for these purposes normally means sequenceidentity of at least 60%, more preferably at least 70%, 80%, 90%, andmost preferably at least 95%. Polypeptides which are “substantiallysimilar” share sequences as noted above except that residue positionswhich are not identical may differ by conservative amino acid changes.

Another indication that nucleotide sequences are substantially identicalis if two molecules hybridize to each other under stringent conditions.However, nucleic acids which do not hybridize to each other understringent conditions are still substantially identical if thepolypeptides which they encode are substantially identical. This mayoccur, e.g., when a copy of a nucleic acid is created using the maximumcodon degeneracy permitted by the genetic code. One indication that twonucleic acid sequences are substantially identical is that thepolypeptide, which the first nucleic acid encodes, is immunologicallycross reactive with the polypeptide encoded by the second nucleic acid.

(e) (ii) The terms “substantial identity” in the context of a peptideindicates that a peptide comprises a sequence with at least 70% sequenceidentity to a reference sequence, preferably 80%, more preferably 85%,most preferably at least 90% or 95% sequence identity to the referencesequence over a specified comparison window. Preferably, optimalalignment is conducted using the homology alignment algorithm ofNeedleman and Wunsch, J. Mol. Biol. 48: 443 (1970). An indication thattwo peptide sequences are substantially identical is that one peptide isimmunologically reactive with antibodies raised against the secondpeptide. Thus, a peptide is substantially identical to a second peptide,for example, where the two peptides differ only by a conservativesubstitution.

The terms “stringent conditions” or “stringent hybridization conditions”includes reference to conditions under which a probe will hybridize toits target sequence, to a detectably greater degree than other sequences(e.g., at least 2-fold over background). Stringent conditions aresequence-dependent and will be different in different circumstances. Bycontrolling the stringency of the hybridization and/or washingconditions, target sequences can be identified which are 100%complementary to the probe (homologous probing). Alternatively,stringency conditions can be adjusted to allow some mismatching insequences so that lower degrees of similarity are detected (heterologousprobing). Generally, a probe is less than about 1000 nucleotides inlength, optionally less than 500 nucleotides in length.

Typically, stringent conditions will be those in which the saltconcentration is less than about 1.5 M Na ion, typically about 0.01 to1.0 M Na ion concentration (or other salts) at pH 7.0 to 8.3 and thetemperature is at least about 30° C. for short probes (e.g., 10 to 50nucleotides) and at least about 60° C. for long probes (e.g., greaterthan 50 nucleotides). Stringent conditions may also be achieved with theaddition of destabilizing agents such as formamide. Exemplary lowstringency conditions include hybridization with a buffer solution of 30to 35% formamide, 1 M NaCl, 1% SDS (sodium dodecyl sulphate) at 37° C.,and a wash in 1× to 2×SSC (20×SSC=3.0 M NaCl/0.3 M trisodium citrate) at50 to 55° C. Exemplary moderate stringency conditions includehybridization in 40 to 45% formamide, 1 M NaCl, 1% SDS at 37° C., and awash in 0.5× to 1×SSC at 55 to 60° C. Exemplary high stringencyconditions include hybridization in 50% formamide, 1 M NaCl, 1% SDS at37° C., and a wash in 0.1×SSC at 60 to 65° C.

Specificity is typically the function of post-hybridization washes, thecritical factors being the ionic strength and temperature of the finalwash solution. For DNA-DNA hybrids, the T_(m) can be approximated fromthe equation of Meinkoth and Wahl, Anal. Biochem., 138:267-284 (1984):T_(m)=81.5° C.+16.6 (log M)+0.41 (%GC)−0.61 (% form)−500/L; where M isthe molarity of monovalent cations, %GC is the percentage of guanosineand cytosine nucleotides in the DNA, % form is the percentage offormamide in the hybridization solution, and L is the length of thehybrid in base pairs. The T_(m) is the temperature (under defined ionicstrength and pH) at which 50% of a complementary target sequencehybridizes to a perfectly matched probe. T_(m) is reduced by about 1° C.for each 1% of mismatching; thus, T_(m), hybridization and/or washconditions can be adjusted to hybridize to sequences of the desiredidentity. For example, if sequences with ≧90% identity are sought, theT_(m) can be decreased 10° C. Generally, stringent conditions areselected to be about 5° C. lower than the thermal melting point (T_(m))for the specific sequence and its complement at a defined ionic strengthand pH. However, severely stringent conditions can utilize ahybridization and/or wash at 1, 2, 3, or 4° C. lower than the thermalmelting point (T_(m)); moderately stringent conditions can utilize ahybridization and/or wash at 6, 7, 8, 9, or 10° C. lower than thethermal melting point (T_(m)); low stringency conditions can utilize ahybridization and/or wash at 11, 12, 13, 14, 15, or 20° C. lower thanthe thermal melting point (T_(m)). Using the equation, hybridization andwash compositions, and desired T_(m), those of ordinary skill willunderstand that variations in the stringency of hybridization and/orwash solutions are inherently described. If the desired degree ofmismatching results in a T_(m) of less than 45° C. (aqueous solution) or32° C. (formamide solution) it is preferred to increase the SSCconcentration so that a higher temperature can be used. An extensiveguide to the hybridization of nucleic acids is found in Tijssen,Laboratory Techniques in Biochemistry and MolecularBiology—Hybridization with Nucleic Acid Probes, Part I, Chapter 2“Overview of principles of hybridization and the strategy of nucleicacid probe assays”, Elsevier, N.Y. (1993); and Current Protocols inMolecular Biology, Chapter 2, Ausubel, et al., Eds., Greene Publishingand Wiley-Interscience, New York (1995).

The present invention provides isolated nucleic acids comprisingpolynucleotides complementary to the polynucleotides of the ZmRAD51polynucleotides. As those of skill in the art will recognize,complementary sequences base-pair throughout the entirety of theirlength with the polynucleotides (i.e., have 100% sequence identity overtheir entire length). Complementary bases associate through hydrogenbonding in double stranded nucleic acids. For example, the followingbase pairs are complementary: guanine and cytosine; adenine and thymine;and adenine and uracil.

The present invention provides isolated nucleic acids comprisingpolynucleotides of the present invention, wherein the polynucleotidesencode a protein having a subsequence of contiguous amino acids from apolypeptide of the present invention. The length of contiguous aminoacids from the prototype polypeptide is selected from the group ofintegers consisting of from at least 10 to the number of amino acidswithin the prototype sequence. Thus, for example, the polynucleotide canencode a polypeptide having a subsequence having at least 10, 15, 20,25, 30, 35, 40, 45, or 50, contiguous amino acids from the prototypepolypeptide.

The present invention provides subsequences comprising isolated nucleicacids containing at least 15 contiguous bases of the inventivesequences. The number of such subsequences encoded by a polynucleotideof the instant embodiment can be any integer selected from the groupconsisting of from 1 to 20, such as 2, 3, 4, or 5. The subsequences canbe separated by any integer of nucleotides from 1 to the number ofnucleotides in the sequence such as at least 5, 10, 15, 25, 50, 100, or200 nucleotides. Subsequences of the isolated nucleic acid can be usedto modulate or detect gene expression by introducing into thesubsequences compounds which bind, intercalate, cleave and/or crosslingto nucleic acids. Exemplary compounds include acridine, psoralen,phenanthroline, naphthoquinone, daunomycin or chloroethylaminoarylconjugates. The subsequences of the present invention can comprisestructural characteristics of the sequence from which it is derived.Alternatively, the subsequences can lack certain structuralcharacterisitics of the larger sequence from which it is derived such asa poly(A) tail.

The proteins encoded by polynucleotides of this embodiment, whenpresented as an immunogen, elicit the production of polyclonalantibodies which specifically bind to a prototype polypeptide such asthe ZmRAD51 polypeptides. Generally, however, a protein encoded by apolynucleotide of this embodiment does not bind to antisera raisedagainst the prototype polypeptide when the antisera has been fullyimmunosorbed with the prototype polypeptide. Methods of making andassaying for antibody binding specificity/affinity are well known in theart Exemplary immunoassay formats include ELISA, competitiveimmunoassays, radioimmunoassays, Western blots, indirectimmunofluorescent assays and the like.

Nucleotide sequences encoding ZmRAD51 have now been determined bymethods described more fully in the Examples below. Briefly, DNAencoding ZmRAD51 was obtained by screening a maize tassel library with a360 bp probe isolated using a set of degenerate PCR primers designedfrom known RAD51 consensus sequences. The nucleic acid sequences and thecorresponding deduced amino acid sequences for the two ZmRAD51recombinases are shown below in Tables I (SEQ ID NO: 1) and II (SEQ IDNO: 5).

Tables I and II disclose the full nucleotide sequence of the cDNA clonesfor ZmRAD51A and ZmRAD51B, respectively. The ATG start of translation ineach case is indicated in bold, as is the TGA translation stop codon.Both genes are 1020 nucleotides long, coding for polypeptides of 340amino acids. The two maize genes exhibit substantial identity with eachother, primarily in the coding portion; however they do diverge insequence in the untranslated regions, a feature that allowed theidentification of unique sequences suitable for making gene-specific PCRprobes. In comparison to the other RAD51 genes, similarity is also highwith the reported tomato sequence. This similarity, except for conservedregions, decreases when comparisons are made to the animal RAD51 genes.Table V compares the actual % similarity and % identity in thepolypeptide sequences among the different genes. The two maize RAD51recombinases are over 94% similar to each other and 90% identical in thecoding portions. Very similar values are observed when the maizepolypeptide sequences are compared to tomato RAD51. The similarity dropsto about 82% and identity to about 69% when comparing the two maizeRAD51 recombinases to animal RAD51 recombinase sequences.

The two cloned maize cDNAs offer both conserved sequences that can beused to recover other RAD51 related genes, as well as unique sequencessuitable for generating gene- or sequence-specific probes. The twoZmRAD51 cDNAs were cloned into vectors and unique PCR amplifiedfragments were subsequently mapped along with an assortment of otherRFLP probes onto previously constructed maize RFLP maps using differentpopulations generated for this purpose. The vector PHP8057 contains theZmRAD51A cDNA cloned into pBlueScript™ vector. The vector PHP8058contains the ZmRAD51B cDNA cloned into pBlueScrip™. The specificsequences that were PCR amplified from vectors PHP8057 and PHP8058 andused as fragment probes for the mapping work are shown in Tables III(SEQ ID NO: 9) and IV (SEQ ID NO: 10). Only the sense strands are shownin these Tables. The regions corresponding to primers PHN10664 (5′primer for RAD51A, SEQ ID NO: 19), PHN10665 (5′ primer for RAD51B, SEQID NO: 20), and the sequence complement of PHN162 (3′ primer for both)are underlined in the Tables.

The ZmRAD51A gene was mapped in a MARSA (Marker Assisted RecombinantSelection A population) F4 population generated from crosses of maizelines R03×N46. In the RFLP map of the MARSA population, the ZmRAD51Agene mapped to chromosome 7, about 40% down the length of the linkagegroup. In the RFLP map of the ALEB9 population, ZmRAD51B maps onchromosome 3, about 25% down the length of this linkage group. Each ofthe clone fragments mapped to a single locus making them usefulreference markers for those positions on the linkage groups.

Redundancy in the genetic code permits variation in the gene sequencesshown in Table I and Table II. In particular, one skilled in the artwill recognize specific codon preferences by a specific host species andcan adapt the disclosed sequence as preferred for a desired host. Forexample, preferred codon sequences typically have rare codons (i.e.,codons having a usage frequency of less than about 20% in knownsequences of the desired host) replaced with higher frequency codons.Codon preferences for a specific organism may be calculated, forexample, by utilizing codon usage tables available on the INTERNET.Codon usage in the coding regions of the polynucleotides of the presentinvention can be analyzed statistically using commercially availablesoftware packages such as “Codon Preference” available from theUniversity of Wisconsin Genetics Computer Group (see Devereaux et al.,Nucleic Acids Res. 12: 387-395 (1984)) or MacVector 4.1 (Eastman KodakCo., New Haven, Conn.). Thus, the present invention provides a codonusage frequency characteristic of the coding region of at least one ofthe polynucleotides of the present invention. The number ofpolynucleotides that can be used to determine a codon usage frequencycan be any integer from 1 to the number of polynucleotides of thepresent invention as provided herein. Optionally, the polynucleotideswill be full-length sequences. An exemplary number of sequences forstatistical analysis can be at least 1, 5, 10, 20, 50, or 100.Nucleotide sequences which have been optimized for a particular hostspecies by replacing any codons having a usage frequency of less thanabout 20% are referred to herein as “codon optimized sequences. ”

Additional sequence modifications are known to enhance proteinexpression in a cellular host. These include elimination of sequencesencoding spurious polyadenylation signals, exon/intron splice sitesignals, transposon-like repeats, and/or other such well-characterizedsequences which may be deleterious to gene expression. The GC content ofthe sequence may be adjusted to levels average for a given cellularhost, as calculated by reference to known genes expressed in the hostcell. Where possible, the sequence may also be modified to avoidpredicted hairpin secondary mRNA structures. Other useful modificationsinclude the addition of a translational initiation consensus sequence atthe start of the open reading frame, as described in Kozak, Mol. CellBiol., 9:5073-5080 (1989). Nucleotide sequences which have beenoptimized for expression in a given host species by elimination ofspurious polyadenylation sequences, elimination of exon/intron splicingsignals, elimination of transposon-like repeats and/or optimization ofGC content in addition to codon optimization are referred to herein asan “expression enhanced sequence.”

More effective variants of RAD51A or RAD51B could be synthesized throughthe use of in vitro recombination (Zhang, J.-H., G. Dawes, W. P. C.Stemmer. 1997. Directed evolution of a fucosidase from a galactosidaseby DNA shuffling and screening. Proc. Natl. Acad. Sci. USA94:4504-4509). For example, the RAD51A and RAD51B from maize and otherspecies could be recombined using the method of DNA shuffling andscreened or selected for more effective variants.

In addition, the native ZmRAD51 gene or a modified version of theZmRAD51 gene could be further optimized for expression by omitting thepredicted signal and pre-sequence, replacing the signal sequence withanother signal sequence, or replacing the signal and pre-sequence withanother type of targeting or localization sequence. The ZmRAD51 nuclearlocalization sequence is located within in the 5′ end of the codingregion, preferably the first 40 amino acids of sequence SEQ ID NO: 3 or7, more preferably the first 30 amino acids of SEQ ID NO: 3 or 7, evenmore preferably the first 20 amino acids of SEQ ID NO: 3 or 7 or mostpreferably the first 10 amino acids. The corresponding polynucleotidesequence would be from nucleotide 53 to 113 of SEQ ID NO: 1 ornucleotide 73 to 132 of SEQ ID NO: 5 and fragments thereof.

Proteins

The isolated proteins of the present invention comprise a polypeptidehaving at least 10 amino acids encoded by any one of the polynucleotidesof the present invention as discussed more fully, above, or polypeptideswhich are conservatively modified variants thereof. The proteins of thepresent invention or variants thereof can comprise any number ofcontiguous amino acid residues from a polypeptide of the presentinvention, wherein that number is selected from the group of integersconsisting of from 10 to the number of residues in a full-lengthpolypeptide of the present invention. Optionally, this subsequence ofcontiguous amino acids is at least 15, 20, 25, 30, 35, or 40 amino acidsin length, often at least 50, 60, 70, 80, or 90 amino acids in length.Further, the number of such subsequences can be any integer selectedfrom the group consisting of from 1 to 20, such as 2, 3, 4, or 5.

As those of skill will appreciate, the present invention includescatalytically active polypeptides of the present invention (i.e.,enzymes). Catalytically active polypeptides have a specific activity ofat least 20%, 30%, or 40%, and preferably at least 50%, 60%, or 70%, andmost preferably at least 80%, 90%, or 95% that of the native(non-synthetic), endogenous polypeptide. Further, the substratespecificity (K_(cat)/K_(m)) is optionally substantially similar to thenative (non-synthetic), endogenous polypeptide. Typically, the K_(m)will be at least 30%, 40%, or 50%, that of the native (non-synthetic),endogenous polypeptide; and more preferably at least 60%, 70%, 80%, or90%. Methods of assaying and quantifying measures of enzymatic activityand substrate specificity (k_(cat)/K_(m)), are well known to those ofskill in the art.

Generally, the proteins of the present invention will, when presented asan immunogen, elicit production of an antibody specifically reactive toa polypeptide of the present invention. Further, the proteins of thepresent invention will not bind to antisera raised against a polypeptideof the present invention which has been fully immunosorbed with the samepolypeptide. Innunoassays for determining binding are well known tothose of skill in the art. A preferred immunoassay is a competitiveimmunoassay as discussed, infra. Thus, the proteins of the presentinvention can be employed as immunogens for constructing antibodiesimmunoreactive to a protein of the present invention for such exemplaryutilities as immunoassays or protein purification techniques.

Modulating Polypeptide Levels and/or Composition

The present invention further provides a method for modulating (i.e.,increasing or decreasing) the concentration or composition of thepolypeptides of the present invention in a plant or part thereof.Modulation can be effected by increasing or decreasing the concentrationand/or the composition (i.e., the ratio of the polypeptides of thepresent invention) in a plant. The method comprises introducing into aplant cell an expression cassette comprising a polynucleotide of thepresent invention as described above to obtain a transformed plant cell,culturing the transformed plant cell under plant cell growingconditions, and inducing or repressing expression of a polynucleotide ofthe present invention in the plant for a time sufficient to modulateconcentration and/or composition in the plant or plant part.

In some embodiments, the content and/or composition of polypeptides ofthe present invention in a plant may be modulated by altering, in vivoor in vitro, the promoter of a gene to up- or down-regulate geneexpression. In some embodiments, the coding regions of native genes ofthe present invention can be altered via substitution, addition,insertion, or deletion to decrease activity of the encoded enzyme. See,e.g., Kmiec, U.S. Pat. No. 5,565,350; Zarling et al., PCT/US93/03868.And in some embodiments, an isolated nucleic acid (e.g., a vector)comprising a promoter sequence is transfected into a plant cell.Subsequently, a plant cell comprising the promoter operably linked to apolynucleotide of the present invention is selected for by means knownto those of skill in the art such as, but not limited to, Southern blot,DNA sequencing, or PCR analysis using primers specific to the promoterand to the gene and detecting amplicons produced therefrom. A plant orplant part altered or modified by the foregoing embodiments is grownunder plant forming conditions for a time sufficient to modulate theconcentration and/or composition of polypeptides of the presentinvention in the plant. Plant forming conditions are well known in theart and discussed briefly, supra.

In general, concentration or composition is increased or decreased by atleast 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or 90% relative to anative control plant, plant part, or cell lacking the aforementionedexpression cassette. Modulation in the present invention may occurduring and/or subsequent to growth of the plant to the desired stage ofdevelopment. Modulating nucleic acid expression temporally and/or inparticular tissues can be controlled by employing the appropriatepromoter operably linked to a polynucleotide of the present inventionin, for example, sense or antisense orientation as discussed in greaterdetail, supra. Induction of expression of a polynucleotide of thepresent invention can also be controlled by exogenous administration ofan effective amount of inducing compound. Inducible promoters andinducing compounds, which activate expression from these promoters, arewell known in the art. In preferred embodiments, the polypeptides of thepresent invention are modulated in monocots, particularly maize.

Molecular Markers

The present invention provides a method of genotyping a plant comprisinga RAD51 polynucleotide. Preferably, the plant is a monocot, such asmaize or sorghum. Genotyping provides a means of distinguishing homologsof a chromosome pair and can be used to differentiate segregants in aplant population.

Molecular marker methods can be used for phylogenetic studies,characterizing genetic relationships among crop varieties, identifyingcrosses or somatic hybrids, localizing chromosomal segments affectingmonogenic traits, map based cloning, and the study of quantitativeinheritance. See, e.g., Plant Molecular Biology: A Laboratory Manual,Chapter 7, Clark, Ed., Springer-Verlag, Berlin (1997). For molecularmarker methods, see generally, The DNA Revolution by Andrew H. Paterson1996 (Chapter 2) in: Genome Mapping in Plants (ed. Andrew H. Paterson)by Academic Press/R. G. Landis Company, Austin, Tex., pp.7-21.

The particular method of genotyping in the present invention may employany number of molecular marker analytic techniques such as, but notlimited to, restriction fragment length polymorphisms (RFLPs). RFLPs arethe product of allelic differences between DNA restriction fragmentscaused by nucleotide sequence variability. As is well known to those ofskill in the art, RFLPs are typically detected by extraction of genomicDNA and digestion with a restriction enzyme. Generally, the resultingfragments are separated according to size and hybridized with a probe;single copy probes are preferred. Restriction fragments from homologouschromosomes are revealed. Differences in fragment size among allelesrepresent an RFLP. Thus, the present invention further provides a meansto follow segregation of RAD51 genes of the present invention as well aschromosomal sequences genetically linked to RAD51 genes using suchtechniques as RFLP analysis. Linked chromosomal sequences are within 50centiMorgans (cM), often within 40 or 30 cM, preferably within 20 or 10cM, more preferably within 5, 3, 2, or 1 cM of a RAD51 gene of thepresent invention.

In the present invention, the nucleic acid probes employed for molecularmarker mapping of plant nuclear genomes selectively hybridize, underselective hybridization conditions, to a gene encoding a RAD51polynucleotide. In preferred embodiments, the probes are selected fromRAD51 polynucleotides. Typically, these probes are cDNA probes or Pst Igenomic clones. In the present invention probes can be made from thepolynucleotide sequences found in Table III (SEQ ID NO:9), Table IV (SEQID NO: 10), or SEQ ID NO:11. The length of RAD51 probes are typically atleast 15 bases in length, more preferably at least 20, 25, 30, 35, 40,or 50 bases in length. Generally, however, the probes are less thanabout 1 kilobase in length. Preferably, the probes are single copyprobes that hybridize to a unique locus in a haploid chromosomecomplement. Some exemplary restriction enzymes employed in RFLP mappingare EcoRI, EcoRv, and SstI. As used herein the term “restriction enzyme”includes reference to a composition that recognizes and, alone or inconjunction with another composition, cleaves at a specific nucleotidesequence.

The method of detecting an RFLP comprises the steps of (a) digestinggenomic DNA of a plant with a restriction enzyme; (b) hybridizing anucleic acid probe, under selective hybridization conditions, to a RADSpolynucleotide sequence of said genomic DNA; (c) detecting therefrom aRFLP.

Other methods of differentiating polymorphic (allelic) variants of thepolynucleotides of the present invention can be had by utilizingmolecular marker techniques well known to those of skill in the artincluding such techniques as: 1) single stranded conformation analysis(SSCP); 2) denaturing gradient gel electrophoresis (DGGE); 3) RNaseprotection assays; 4) allele-specific oligonucleotides (ASOs); 5) theuse of proteins which recognize nucleotide mismatches, such as the E.coli mutS protein; and 6) allele-specific PCR. Other approaches based onthe detection of mismatches between the two complementary DNA strandsinclude clamped denaturing gel electrophoresis (CDGE); heteroduplexanalysis (HA); and chemical mismatch cleavage (CMC). Thus, the presentinvention further provides a method of genotyping comprising the stepsof contacting, under stringent hybridization conditions, a samplesuspected of comprising a RAD51 polynucleotide with a nucleic acidprobe. Generally, the sample is a plant sample; preferably, a samplesuspected of comprising a maize RADS polynucleotide (e.g., gene, mRNA).The nucleic acid probe selectively hybridizes, under stringentconditions, to a subsequence of a RAD51 polynucleotide comprising apolymorphic marker. Selective hybridization of the nucleic acid probe tothe polymorphic marker nucleic acid sequence yields a hybridizationcomplex. Detection of the hybridization complex indicates the presenceof that polymorphic marker in the sample. In preferred embodiments, thenucleic acid probe comprises a RAD51 polynucleotide.

Expression of Proteins in Host Cells

Using the nucleic acids of the present invention, one may express aprotein of the present invention in a recombinantly engineered cell suchas bacteria, yeast, insect, mammalian, or preferably plant cells. Thecells produce the protein in a non-natural condition (e.g., in quantity,composition, location, and/or time), because they have been geneticallyaltered through human intervention to do so.

It is expected that those of skill in the art are knowledgeable in thenumerous expression systems available for expression of a nucleic acidencoding a protein of the present invention. No attempt to describe indetail the various methods known for the expression of proteins inprokaryotes or eukaryotes will be made. A review of expression systemscan be found in Recombinant Gene Expression Protocols, Tuan, Ed., HumanaPress, New Jersey (1997).

In brief summary, the expression of isolated nucleic acids encoding aprotein of the present invention will typically be achieved by operablylinking, for example, the DNA or cDNA to a promoter (which is eitherconstitutive, cell and/or tissue specific, or inducible), followed byincorporation into an expression vector. The vectors can be suitable forreplication and integration in either prokaryotes or eukaryotes. Typicalexpression vectors contain transcription and translation terminators,initiation sequences, and promoters useful for regulation of theexpression of the DNA encoding a protein of the present invention. Toobtain high level expression of a cloned gene, it is desirable toconstruct expression vectors which contain, at the minimum, a strongpromoter to direct transcription, a ribosome binding site fortranslational initiation, and a transcription/translation terminator.One of skill would recognize that modifications can be made to a proteinof the present invention without diminishing its biological activity.Some modifications may be made to facilitate the cloning, expression, orincorporation of the targeting molecule into a fusion protein. Suchmodifications are well known to those of skill in the art and include,for example, a methionine added at the amino terminus to provide aninitiation site, or additional amino acids (e.g., poly His) placed oneither terminus to create conveniently located restriction sites ortermination codons or purification sequences.

A. Expression in Prokaryotes

Prokaryotic cells may be used as hosts for expression. Prokaryotes mostfrequently are represented by various strains of E. coli; however, othermicrobial strains may also be used. Commonly used prokaryotic controlsequences which are defined herein to include promoters fortranscription initiation, optionally with an operator, along withribosome binding site sequences, include such commonly used promoters asthe beta lactamase (penicillinase) and lactose (lac) promoter systems(Chang et al., Nature 198:1056 (1977)), the tryptophan (trp) promotersystem (Goeddel et al., Nucleic Acids Res. 8:4057 (1980)) and the lambdaderived P L promoter and N-gene ribosome binding site (Shimatake et al.,Nature 292:128 (1981)). The inclusion of selection markers in DNAvectors transfected in E. coli is also useful. Examples of such markersinclude genes specifying resistance to ampicillin, tetracycline, orchloramphenicol.

The vector is selected to allow introduction into the appropriate hostcell. Bacterial vectors are typically of plasmid or phage origin.Appropriate bacterial cells are infected with phage vector particles ortransfected with naked phage vector DNA. If a plasmid vector is used,the bacterial cells are transfected with the plasmid vector DNA.Expression systems for expressing a protein of the present invention areavailable using Bacillus sp. and Salmonella (Palva, et al., Gene 22:229-235 (1983); Mosbach, et al., Nature 302: 543-545 (1983)).

B. Expression in Eukaryotes

A variety of eukaryotic expression systems such as yeast, insect celllines, plant and mammalian cells, are known to those of skill in theart. As explained briefly below, a protein of the present invention canbe expressed in these eukaryotic systems. In some embodiments,transformed/transfected plant cells, as discussed infra, are employed asexpression systems for production of the proteins of the instantinvention.

Synthesis of heterologous proteins in yeast is well known. Sherman, F.,et al., Methods in Yeast Genetics, Cold Spring Harbor Laboratory (1982)is a well recognized work describing the various methods available toproduce the protein in yeast. Suitable vectors usually have expressioncontrol sequences, such as promoters, including 3-phosphoglyceratekinase or other glycolytic enzymes, and an origin of replication,termination sequences and the like as desired. For instance, suitablevectors are described in the literature (Botstein, et al., Gene 8: 17-24(1979); Broach, et al., Gene 8: 121-133 (1979)).

A protein of the present invention, once expressed, can be isolated fromyeast by lysing the cells and applying standard protein isolationtechniques to the lysates. The monitoring of the purification processcan be accomplished by using Western blot techniques or radioimmunoassayof other standard immunoassay techniques.

The sequences encoding proteins of the present invention can also beligated to various expression vectors for use in transfecting cellcultures of, for instance, mammalian, insect, or plant origin.Illustrative of cell cultures useful for the production of the peptidesare mammalian cells. Mammalian cell systems often will be in the form ofmonolayers of cells although mammalian cell suspensions may also beused. A number of suitable host cell lines capable of expressing intactproteins have been developed in the art, and include the HEK293, BHK21,and CHO cell lines. Expression vectors for these cells can includeexpression control sequences, such as an origin of replication, apromoter (e.g., the CMV promoter, a HSV tk promoter or pgk(phosphoglycerate kinase) promoter), an enhancer (Queen et al., Immunol.Rev. 89: 49 (1986)), and necessary processing information sites, such asribosome binding sites, RNA splice sites, polyadenylation sites (e.g.,an SV40 large T Ag poly A addition site), and transcriptional terminatorsequences. Other animal cells useful for production of proteins of thepresent invention are available, for instance, from the American TypeCulture Collection Catalogue of Cell Lines and Hybridomas (7th edition,1992).

Appropriate vectors for expressing proteins of the present invention ininsect cells are usually derived from the SF9 baculovirus. Suitableinsect cell lines include mosquito larvae, silkworm, armyworm, moth andDrosophila cell lines such as a Schneider cell line (See Schneider, J.Embryol. Exp. Morphol. 27: 353-365 (1987).

As with yeast, when higher animal or plant host cells are employed,polyadenlyation or transcription terminator sequences are typicallyincorporated into the vector. An example of a terminator sequence is thepolyadenlyation sequence from the bovine growth hormone gene. Sequencesfor accurate splicing of the transcript may also be included. An exampleof a splicing sequence is the VP1 intron from SV40 (Sprague, et al., J.Virol. 45: 773-781 (1983)). Additionally, gene sequences to controlreplication in the host cell may be incorporated into the vector such asthose found in bovine papilloma virus type-vectors. Saveria-Campo, M.,Bovine Papilloma Virus DNA a Eukaryotic Cloning Vector in DNA CloningVol. II a Practical Approach, D. M. Glover, Ed., IRL Press, Arlington,Va. pp. 213-238 (1985).

Gene Delivery

An isolated maize RAD51 recombinase gene may be incorporated into aplasmid and introduced into a host cell, e.g., a heterologous non-maizecell. Expression of the recombinant ZmRAD51 protein encoded by one ofthe nucleotide sequences disclosed herein can provide a source of asubstantially pure plant recombinase.

A polynucleotide sequence encoding for ZmRAD51A (SEQ ID NO:2) orZmRAD51B (SEQ ID NO:6) may be delivered to a host cell such as a plantcell for transient transformation or stable integration into the plant'sgenome by methods known in the art. Preferably, the host cell is a plantcell and, more preferably, a monocot cell, such as a maize cell. Toaccomplish such delivery, a nucleotide sequence containing the codingsequence for ZmRAD51A (SEQ ID NO:2) or the coding sequence for ZmRAD51B(SEQ ID NO:6) may be attached to regulatory elements needed for theexpression of the gene in a particular host cell or system. Theseregulatory elements include, for example, promoters, terminators, andother elements that permit desired expression of the enzyme in aparticular plant host, in a particular tissue or organ of a host such asvascular tissue, root, leaf, or flower, or in response to a particularsignal. These regulatory elements may also include the native regulatorysequences normally associated with the RAD51 genes in their endogenousstate.

Promoters

A promoter is a DNA sequence that directs the transcription of astructural gene, e.g., that portion of the DNA sequence that istranscribed into messenger RNA (mRNA) and then translated into asequence of amino acids characteristic of a specific polypeptide.Typically, a promoter is located 5′ of the structural gene it controls,proximal to the transcriptional start site. A promoter may be inducible(or derepressible), increasing the rate of transcription in response tothe presence or absence of a resulting agent. In contrast, a promotermay be constitutive, whereby the rate of transcription is not regulatedby a specific agent. A promoter may be regulated in a tissue-specific ortissue-preferred manner, such that it is only active in transcribing theoperably linked coding region in a specific tissue type or types, suchas plant leaves, roots, or meristem. Examples of suitable promoterswhich may be operably linked to the present ZmRAD51 coding sequencesinclude the maize ubiquitin promoter ubiquitin (Christensen et al.,Plant Mol. Biol., 12:619-632 (1992) and the ZmDJ1 promoter (Baszczynskiet al., Maydica, 42:189-201 (1997)).

Inducible Promoters

An inducible promoter useful in the present invention may be operablylinked to a nucleotide sequence encoding ZmRAD51. Optionally, theinducible promoter is operably linked to a nucleotide sequence encodinga signal sequence which is operably linked to a nucleotide sequenceencoding ZmRAD51. With an inducible promoter, the rate of transcriptionincreases in response to an inducing agent. Any inducible promoter canbe used in the present invention to direct transcription of ZmRAD51,including those described in Ward et al., Plant Molecular Biol., 22:361:-366 (1993). Exemplary inducible promoters include that from theACE1 system which responds to copper (Mett et al., Proc. Nat'l Acad.Sci. (U.S.A.) NAS, 90: 4567-4571(1993)); the In 2 gene promoter frommaize which responds to benzenesulfonamide herbicide safeners (Hersheyand Stoner, Plant Mol. Biol., 17:679-690 (1991)); and the Tet repressorfrom Tn10 (Hershey, Mol. Gen. Genetics, 227:229-237 (1991)).

A particularly preferred inducible promoter is one that responds to aninducing agent to which plants do not normally respond. One example ofsuch a promoter is the steroid hormone gene promoter. Transcription ofthe steroid hormone gene promoter is induced by a glucocorticosteroidhormone. (Schena et al., Proc. Nat'l. Acad. Sci. (U.S.A.)., 88:10421(1991)).

The present invention also provides an expression vector having aninducible promoter operably linked to a nucleotide sequence encodingZmRAD51. The expression vector may be introduced into plant cells andthe cells exposed to an inducer of the inducible promoter. The cells maythen be screened for the presence of ZmRAD51 protein by immunoassaymethods.

Tissue-specific or Tissue-Preferred Promoters

An expression vector of the present invention may include atissue-specific or tissue-preferred promoter operably linked to thenucleotide sequence encoding ZmRAD51. The expression vector isintroduced into plant cells. The cells may be screened for the presenceof ZmRAD51 protein, e.g., by immunological methods.

Optionally, the tissue-preferred promoter is operably linked to anucleotide sequence encoding a signal sequence which is operably linkedto a nucleotide sequence encoding ZmRAD51. Plants transformed with agene encoding ZmRAD51 operably linked to a tissue specific promoterproduce ZmRAD51 protein at least preferentially and, preferably,exclusively (“tissue-specific promoter”) in a specific tissue.

Any tissue-specific or tissue-preferred promoter can be utilized in theinstant invention. Examples of such promoters include a root-preferredpromoter such as that from the phaseolin gene as described inSengupta-Gopalan et al., Proc. Nat'l, Acad. Sci. (U.S.A.), 82:3320-3324(1985) or the TobRB7 gene characterized by Yamamoto et al, Plant Cell,3:371-382 (1991); a leaf-specific and light-induced promoter such asthat from cab or rubisco as described in Simpson et al., EMBO J.,4(11):2723-2729 (1985); an anther-specific promoter such as that fromLAT52 as described in Twell et al., Mol. Gen. Genet., 217:240-245(1989); a pollen-specific promoter such as that from Zm13 as describedin Guerrero et al., Mol. Gen. Genet., 224:161-168 (1993); and amicrospore-preferred promoter such as that from apg as described inTwell et al., Sex. Plant Reprod., 6:217-224 (1993).

Other tissue-specific promoters useful in the present invention includea phloem-preferred promoter such as that associated with the Arabidopsissucrose synthase gene as described in Martin et al., 1993, The PlantJournal 4:367-377; a floral-specific promoter such as that of theArabidopsis HSP 18.2 gene described in Tsukaya et al., Mol. Gen. Genet.,237:26-32 (1993) and of the Arabidopsis HMG2 gene as described in Enjutoet al., Plant Cell, 7:517-527 (1995).

Constitutive Promoters

Alternatively, the nucleotide sequence encoding ZmRAD51 may be operablylinked to a constitutive promoter. Optionally, the constitutive promoteris operably linked to a nucleotide sequence encoding a signal sequencewhich is operably linked to a nucleotide sequence encoding ZmRAD51. Manydifferent constitutive promoters can be utilized in the instantinvention to express ZmRAD51. Examples include promoters from plantviruses such as the 35S promoter from cauliflower mosaic virus (CaMV),as described in Odell et al., Nature, 313:810-812 (1985), and promotersfrom genes such as rice actin (McElroy et al., Plant Cell, 2:163-171(1990)); ubiquitin (Christensen et al., Plant Mol. Biol., 12:619-632(1992); pEMU (Last et al., Theor. Appl. Genet., 81:581-588 (1991)); MAS(Velten et al., EMBO J., 3:2723-2730 (1984)); and maize H3 histone(Lepetit et al., Mol. Gen. Genet., 231:276-285 (1992)).

Additional Regulatory Elements

Additional regulatory elements that may be connected to the ZmRAD51nucleic acid sequence for expression in plant cells include terminators,polyadenylation sequences, and nucleic acid sequences encoding signalpeptides that permit localization within a plant cell or secretion ofthe protein from the cell. Such regulatory elements and methods foradding or exchanging these elements with the regulatory elements of theZmRAD51 gene are known, and include, but are not limited to,3′termination and/or polyadenylation regions such as those of theAgrobacterium tumefaciens nopaline synthase (nos) gene (Bevan et al.,Nucl. Acids Res., 12:369-385 (1983)); the potato proteinase inhibitor II(PinII) gene (Keil et al., Nucl. Acids Res., 14:5641-5650 (1986)); andthe CaMV 19S gene (Mogen et al., Plant Cell, 2:1261-1272 (1990)).

Plant signal sequences, including, but not limited to, signal-peptideencoding DNA/RNA sequences which target proteins to the extracellularmatrix of the plant cell (Dratewka-Kos et al., J. Biol. Chem.,264:48964900 (1989)) and the Nicotiana plumbaginifolia extensin gene(DeLoose et al., Gene, 99:95-100 (1991)), or signal peptides whichtarget proteins to the vacuole like the sweet potato sporamin gene(Matsuka et al., Proc. Nat'l. Acad. Sci. (U.S.A.), 88:834 (1991)) andthe barley lectin gene (Wilkins et al., Plant Cell, 2:301-313 (1990)),or signals which cause proteins to be secreted such as that of PRIb(Lind et al., Plant Mol. Biol., 18:47-53 (1992)), or those which targetproteins to the plastids such as that of rapeseed enoyl-ACP reductase(Verwaert et al., Plant Mol. Biol., 26:189-202 (1994)) are useful in theinvention.

Another regulatory element that may be employed in combination with theZmRAD51 nucleic acid sequence for expression in plant cells is a nuclearlocalization sequence (“NLS”) which directs localization of expressionof the ZmRAD51 protein to the nucleus of a plant cell. Examples ofsuitable nuclear localization sequences may be found in Kalderon et al.,Cell, 39:499-509 (1984) and Hicks et al., Plant Cell, 5:983-994 (1993).Alternatively, the native ZmRAD51 nuclear localization signal located inthe 5′ region of the coding sequence, most preferably from nucleotide 53to 113 of SEQ ID NO: 1 or nucleotide 73 to 132 of SEQ ID NO:5 could beused.

Gene Delivery Methods

Numerous methods for introducing foreign genes into plant cells areknown and can be used to insert a ZmRAD51 gene into a plant host cell,including biological and physical DNA delivery protocols. See, forexample, Miki et al., “Procedure for Introducing Foreign DNA intoPlants”, in: Methods in Plant Molecular Biology and Biotechnology, Glickand Thompson, eds., CRC Press, Inc., Boca Raton, pages 67-88 (1993). Themethods chosen vary with the host plant, and include chemicaltransfection methods such as calcium phosphate, microorganism-mediatedgene transfer such as Agrobacterium (Horsch et al., Science, 227:1229-31(1985)), electroporation, micro-injection, and biolistic bombardment.

Expression cassettes and vectors and in vitro culture methods for plantcell or tissue transformation and regeneration of plants are also knownand available. See, for example, Gruber et al., “Vectors for PlantTransformation,” in: Methods in Plant Molecular Biology andBiotechnology, Glick and Thompson, eds., CRC Press, Inc., Boca Raton,pages 89-119 (1993). As used herein, an “expression cassette” is anucleic acid construct, generated recombinantly or synthetically, with aseries of specified nucleic acid elements which permit transcription ofa particular nucleic acid in a host cell. The expression cassette can beincorporated into a plasmid, chromosome, mitochondrial DNA, plastid DNA,virus, or nucleic acid fragment. Typically, the expression cassetteportion of an expression vector includes, among other sequences, anucleic acid to be transcribed, and a promoter.

Agrobacterium-mediated Gene Delivery

One widely utilized method for introducing an expression vector intoplants is based on the natural transformation system of Agrobacterium.A. tumefaciens and A. rhizogenes are plant pathogenic soil bacteria thatgenetically transform plant cells. The Ti and Ri plasmids of A.tumefaciens and A. rhizogenes, respectfully, carry genes responsible forgenetic transformation of plants (see, e.g., Kado, Crit. Rev.Plant Sci.,10: 1 (1991)). Descriptions of the Agrobacterium vector systems andmethods for Agrobacterium-mediated gene transfer are provided in Gruberet al., supra, see also Hiei, et al., U.S. Pat. No. 5,591,616, issuedJan. 7, 1997.

Direct Gene Transfer

Despite the fact that the host range for Agrobacterium-mediatedtransformation is broad, some major cereal crop species and gymnospermshave generally been recalcitrant to this mode of gene delivery, eventhough some success has recently been achieved in rice and maize (Hieiet al., The Plant Journal, 6:271-282 (1994); Ishida et al., NatureBiotechnology, 14:745-750 (1996), Hiei, et al., U.S. Pat. No. 5,591,616,issued Jan. 7, 1997). Several other methods of introducing foreign DNAinto plant cells, collectively referred to as direct gene transfer, havebeen developed as an alternative to Agrobacterium-mediated delivery.

A generally applicable method of delivering DNA into plant cells ismicroprojectile-mediated delivery, where DNA is carried on the surfaceof microprojectiles measuring about 1 to 4 μM in diameter. Theexpression vector is introduced into plant tissues with a biolisticdevice that accelerates the microprojectiles to speeds of 300 to 600 m/swhich is sufficient to penetrate the plant cell walls and membranes.(e.g., Klein et al., Biotechnology, 10:268 (1992)).

Another method for physical delivery of DNA to plants is sonication oftarget cells as described in Zang et al., Bio/Technology, 9:996 (1991).Alternatively, liposome or spheroplast fusions have been used tointroduce expression vectors into plants (see, e.g., Christou et al.,Proc. Nat'l Acad. Sci. (U.S.A.), 84:3962 (1987)). Direct uptake of DNAinto protoplasts using CaCl₂ precipitation , polyvinyl alcohol orpoly-L-ornithine have also been reported. (See, for example, Hain etal., Mol. Gen. Genet., 199:161 (1985)). Electroporation of protoplastsand whole cells and tissues has also been described (see, for example,Spencer et al., Plant Mol.Biol., 24:51-61 (1994)).

Particle Wounding/Agrobacterium Delivery

Another useful basic transformation protocol involves a combination ofwounding by particle bombardment, followed by use of Agrobacterium forDNA delivery, as described by Bidney et al., Plant Mol. Biol.,18:301-313 (1992). Useful plasmids for this delivery method are onescontaining a Bin 19 backbone (see Bevan, Nucleic Acids Research,12:8711-8721 (1984)). This method is commonly used to deliverheterologous DNA into sunflower cells.

Assay Methods

Transgenic plant cells, callus, tissues, shoots, and transgenic plantsmay be tested for a presence of the ZmRAD51 gene by DNA analysis and forexpression of the gene by immunoassay. For example, the presence of aZmRAD51 gene can be confirmed by Southern analysis. This commonprocedure may be carried out by isolating DNA from the cells inquestion, cutting the DNA using restriction enzymes, fractionating theresulting DNA fragments on an agarose gel to separate the fragments bymolecular weight. The separated DNA fragments are then transferred tonitro-cellulose membranes and hybridized with a radioactively labeledprobe fragment (e.g., labeled with ³²P) and washed with an SDS solution(see, e.g., Southern, J. Molec. Biol., 98:503-517 (1975)).Alternatively, the presence of the ZmRAD51 gene in transgenic cells maybe verified by amplifying the gene (or a portion of the gene) bypolymerase chain reaction (“PCR”) using appropriate primers, cutting theDNA from the PCR with restriction enzymes, and fractionating theresulting DNA fragments as described above and detecting the PCRamplified DNA fragment with an appropriate probe (see, e.g., Saiki,Science, 239:487-491 (1988)).

Instead of examining the transgenic cells for the presence of theZmRAD51 gene, expression of the gene by the transgenic cell may beprobed using an immunoassay technique to establish the presence ofexpressed ZmRAD51 protein. This is typically carried out by probing theprotein fraction from the transgenic cells with an antibody specific forthe ZmRAD51 protein. The presence of the resulting ZmRAD51protein/antibody complex can be detected using a variety of well knowntechniques.

The invention is further characterized by the following examples. Theseexamples are not meant to limit the scope of the invention as set forthin the foregoing description and variations within the concepts of theinvention will be apparent.

EXAMPLES Example 1 Cloning of ZmRAD51A & ZmRAD51B cDNA

A. Recovery of a ZmRAD51 Probe Fragment

Poly-A mRNA from maize (cv. A632) tassels was prepared using theMicroQuick kit (Pharmacia, Piscataway, N.J.). Room temperature PCR wasperformed on the mRNA using a set of degenerate primers designed fromknownRAD51 consensus sequences. The PCR amplified fragment was clonedand sequenced, and confirmed to be a 360 bp cDNA sequence for a RAD51homolog. The probe fragment clone corresponding to the Zea mays cDNA wasdesignated PHP7763 and had the following sequence:

ACATTCAGACCACAAAGGCTCTTGCAGATTGCTGACAGGTTTGGACTGAATGGTGCTGATGTGTTAGAGAATGTGGCTTATGCCAGAGCTTATAATACGGATCATCAATCTAGACTTCTGCTGGAAGCAGCTTCCATGATGATAGAGACCAGGTTTACTCTTATGGTTGTAGACAGTGCCACAGCTCTGTACAGAACTGATTTCTCAGGAAGAGGGGAACTATCAGCGAGGCAAATGCACATGGCTAAGTTCCTGAGGAGCCTTCAGAAGTTAGCTGATGAGTTTGGAGTAGCTGTGGTTATCACCAATCAAGTAGTGGCCCAAGTGGATGGATCTACTATGTTTGCTGGGCCGCAGTTC(SEQ ID NO: 11).

The PHP7763 probe sequence was used to design and synthesize two maizesequence-specific oligonucleotide primers, PHN7443 having the nucleotidesequence 5′-TATAGAATTCCACAAAGGCTCTTGCAGATTGCTGACAG (SEQ ID NO: 12) andPHN7444 having the sequence,

5′-ATACTCGAGGCCCAGCAAACATAGTAGATCCATCCAC (SEQ ID NO:13).

B. Lambda Library Screening

A lambda cDNA library made by Stratagene (LaJolla, Calif.) from suppliedmaize (cv. A632) tassel RNA was screened using standard procedures asdescribed in Molecular Cloning: A Laboratory Manual, Second Edition,Sambrook, Fritsch and Maniatis, Cold Spring Harbor Laboratory Press,(1989). The library was plated at approximately 50,000 plaques per 150mm plate, transferred to filters (Magnalift brand, MSI, Inc.Westborough, Mass.), and screened using a digoxigenin-dUTP-labeled PCRamplified probe generated using oligonucleotide primers PHN7443 andPHN7444. Labeling and hybridization conditions were as described in theGenius™ System manual by the manufacturer (Boerhinger Mannheim Corp.,Indianapolis, Ind.), with modifications. The full protocols used arelisted in Examples 3 and 4.

The filter probing resulted in 32 spots which aligned well on thelumigraphs from duplicate lifts, 11 more with less optimal alignment, 8with fair aligmnent and 23 with no corresponding alignment on theduplicate lift. The corresponding plaques were picked for furtherevaluation. Additional PCR amplification reactions designed to eliminatefalse positives were carried out using the PHN7443 and PHN7444 primersabove plus M13 forward (PHN162, 5′-TCCCAGTCACGACGTTGTAAAACG SEQ ID NO:14); and reverse (PHN487, 5′-AGCGGATAACAATTTCACACAGGAAACAGCTATGAC SEQ IDNO: 15) primers. Six plaque picks were recovered, titered, replated togive several hundred plaques per plate and lifted and reprobed asdescribed above. Confirmed positive plaques were processed through an invivo “pop-out” protocol that generates phagemid (plasmid) DNA frominfecting lambda phage. The protocol used was developed at Stratagene(La Jolla, Calif.) and is described in the directions that accompanytheir Lamda Zap libraries. When successfully performed, this procedureyields E. coli colonies that contain a pBlueScript™ (Stratagene, LaJolla, Calif.) type plasmid that has been excised from the lambda phage.Each plaque pick used yielded several colonies following this protocol.Additional screening of the plasmid clones resulted in three uniqueclones, as determined by restriction enzyme analysis. Partial sequencingand comparison to known higher eukaryotic RAD51 genes confirmed thatthese clones corresponded to maize homologs of RAD51. Of the threeclones, two were identical in sequence except that one of the two had alonger 3′ untranslated region and was truncated in the coding region.The third clone shared very high identity in the coding region with thefirst two clones, but differed in the untranslated regions. The threeclones were designated PHP7981, PHP7982 and PHP7983. PHP7981 (ZmRAD51A)and PHP7983 (ZmRAD51B) were completely sequenced (see Tables I and IIherein). The RAD51A 3′ untranslated region can be seen in SEQ ID NO:4(nucleotide 1078 to 1538) and the RAD51B 3′ untranslated region can beseen in SEQ ID NO:8 (nucleotide 1099 to 1556).

In order to facilitate later cloning of the ZmRAD51 genes, site-directedmutagenesis (by the method of Su et al., Gene, 69:81-89 (1988)) was usedto introduce restriction sites flanking the coding sequences. A HpaIrestriction site was introduced downstream of the stop codon of PHP7981(position shown in Table I) using the oligonucleotide primer PHN9611 (5′-GTATTGCAGATGTTAAGGATTGAGACCATACCTGGTTAACAGGCATCTCAG3′-SEQ ID NO: 16)to create the plasmid PHP8057. ABamHI site was introduced 5′ to thestart codon of PHP7983 (position shown in Table II) with the primerPHN9612 (5′-GCAGCCAGGGATCCAC-ATGTCCTCGTC3′-SEQ ID NO: 17), and a HpaIsite was inserted 3′ to the stop codon (position shown in Table II) witholigonucleotide PHN9613(5′-CTGATGTCAAGGACTGAAAGCATCCTCATTTGCAGTTAACAGCATAACTTGC3′-SEQ ID NO:18)to create the plasmid PHP8058. These newly created clones served assources of probes for mapping studies.

Example 2 Mapping of the Maize ZmRAD51 Clones

For ZmRAD51 sequence-specific hybridization, oligonucleotide primershomologous to unique sequences in the 3′ untranslated regions ofZmRAD51A (PHN10664; 5′-CCATACCTGCTTTACAGGCATC3′-SEQ ID NO:19) or ofZmRAD51B (PHN10665; 5′-CATCCTCATTTGSAGTCCACAG3′-SEQ ID NO:20; where “S”denotes a mixture of “C” and “G”) were synthesized and used inconjunction with an M13 universal sequencing primer (PHN162;5′-TCCCAGTCACGACGTTGTAAAACG3′ SEQ ID NO: 14) to PCR amplify probefragments from the two vectors PHP8057 (ZmRAD51A) and PHP8058(ZmRAD51B). Sequences of PHN10664 and PHN10665 span the regionsmutagenized to create the HpaI sites, but themselves correspond to thesequences of the original clones in PHP7981 and PHP7983. There wasenough identity to permit efficient PCR amplification using PHP8057 andPHP8058 as templates. This approach was used in order to generate finalprobe fragments identical to the original ZmRAD51A and ZmRAD51B genes.These fragments, which extend from just downstream of the translationstop codon to the end of the poly(A) tail of the cDNA sequences, weresubsequently used as probes against two maize populations and mappositions were determined.

Southern hybridizations were carried out using two different maizepopulations generated as part of a breeding program. Population 1(MARSA), an F4, was generated from crosses of the lines R03×N46, andincluded 200 individuals as part of the mapping family. Population 2(ALEB9), an F2, was generated from crosses of the lines R67×P38 andcontained 240 individuals. DNA was isolated from each individual by aCTAB extraction method (Saghai-Maroof et al., Proc. Nat'l Acad. Sci.(U.S.A.), 81:8014-8018 (1994)) and digested individually withrestriction enzymes BamHI, HindIII, EcoRI and EcoRV. Digests wereseparated on agarose gels and transferred to membranes (Southern, J.Molec. Biol., 98:503-517 (1975)) prior to hybridization (Helentjaris etal., Plant Mol. Biol., 5:109-118 (1985)) with an array of probes toestablish the basic RFLP map. Population 1 membranes were hybridizedusing 179 RFLP probes, while population 2 membranes were hybridizedusing 115 RFLP probes. After hybridization the membranes were exposed tox-ray film for an appropriate length of time to be visually scored. Alldata were entered into an electronic database and map positions of theRFLP probes (Evola et al., Theor. Appl. Genet., 71:765-771(1986)) weredetermined using MAPMAKER (Lincoln et al., in Constructing GeneticLinkage Maps with MAPMARKER/EXP Version 3.0: A Tutorial and ReferenceManual, Whitehead Institute for Biomedical Research, Cambridge, Mass.(1993)) and a map was constructed for each population. Table VI liststhe positions of a number of markers, including the ZmRAD51A gene,mapped on the MARSA population. Table VII lists the positions of anumber of markers, including the ZmRAD51B gene, mapped on the ALEB9population.

Example 3 Hybridization Procedure

A prehybridization solution was prepared containing the followingcomponents:

1% BMB Blocking reagent

1% gelatin

0.2% SDS

0.1% Sarkosyl (n-lauryl sarkosine)

5×SSC (750 mM sodium chloride, 75 mM sodium citrate, pH 7.0)

The solution was heated to facilitate the dissolution of the blockingreagent and the gelatin. After being wet with 2×SSC, filters were placedin the prehybridization solution and incubated at 68° C. for 2 hrs withshaking.

Hybridization was carried out by a procedure which included denaturing aDigoxigenin labeled probe by boiling for 10 minutes and then plunginginto an ice water bath. The probe was added to give 10 to 20 ng of probeper ml of prehybridization solution and mixed well to form ahybridization solution. If the hybridization solution was to be reused,it was heated to 95° C. for 10 minutes. The equilibrated filters wereincubated overnight at 68° C., with gentle shaking or other form ofagitation.

The incubated filters were washed for 5 minutes in a dish with2×SSC+0.1% SDS. This wash was repeated two additional times. The filterswere then washed two times for 1.5 hrs in 0.5×SSC+0.1% SDS at 60-65 C.using pre-warmed wash solution.

Example 4 Labeling Procedure

A. Antibody Probing

Antibody probing was conducted by performing the following steps withindividual filters in Petri dishes. A Genius™ blocking solution wasprepared by dissolving 1% BMB Block, 1% Gelatin and 0.5% Tween 20 inGenius™ 1 buffer. The Genius™ 1 buffer was heated to dissolve theblocking reagent and gelatin, then cooled to room temperature.

Filters were washed 3 times in Genius™ 1 Buffer (5 minutes per wash) andthen incubated 1 hr in Genius™ Blocking solution, with gentle rocking.An anti-digoxigenin/alkaline phosphate conjugate was diluted 1:100 inleftover block (directly into the blocking solution on the filters).After incubation for 0.5 hour, the filters were washed 2 times for 15minutes in Genius™ 1 Buffer.

B. Chemiluminescent Read-out

Without being allowed to dry, filters were washed 2 times in Genius™Buffer 3 (15 minutes per wash). At the start of the first wash,LumiPhos530 was taken out of the refrigerator. X-Ray cassettes wereprepared by placing transparency sheets (3M AF4300 sheets) in thecassettes and taping them in place. About 6 mL of LumiPhos530 was placedinto a Petri dish lid, taking care to handle the LumiPhos aseptically.Filters were removed from the Genius™ Buffer 3 and most of the bufferwas wicked onto Whatman 3 mm paper by just touching the filter's edge tothe Whatman paper. The filter was then placed plaque side down onto theLumiPhos530, ensuring good coverage on the plaque side and allowing mostof the LumiPhos to drain off back into the lid. The filter was thenplaced in the X-ray cassette, plaque side up, on the transparencysheet(s). As soon as a transparency sheet was full, another sheet wasplaced on top of it and bubbles were smoothed out. The top sheet wastaped in place. Putting the top sheet in place quickly prevented thefilters from drying out. The filters in the transparency sheet wereexposed to a Kodak XAR-5 film sheet for 1 to 1.5 hrs. This first filmwas developed and another XAR-5 sheet was exposed overnight. Theovernight sheet was very overexposed, but the stab marks showed as whitespots, allowing ready alignment of the plate to the lumigraph.

C. Picking Plaques

Tubes were prepared with SM buffer (100 mM NaCl, 8 mM MgSO₄-7H₂O, 50 mMTris-Cl, pH 7.5, 0.01% (w/v) gelatin), usually about 1 mL. Stab marks onthe plates were aligned to the marks on the lumigraph. Plaques ofinterest were picked by poking a pipette into the agar and remove a plugcontaining the plaque using either the back of a 5 mL pipette (for 1°picks) or a transfer pipette (for 2° picks). Each plug was added to atube containing the SM buffer. One drop of chloroform (free of isoamylalcohol) was then added to the buffer. After capping and vortexing well,the tube was then incubated at room temperature for 1 to 2 hours, andthen stored at 4° C.

Example 5 Creation of ZmRAD51—Containing Plant Transformation Vectors

Constructs for plant transformation experiments were created in whichthe ZmRAD51A or ZmRAD51B genes were inserted behind a maize ubiquitinpromoter (Christensen et al., Plant Mol. Biol, 18:1185-1187 (1992)). Tofacilitate cloning the two ZmRAD51 genes as BamHI/Hpal fragments, aBamHI site was created 5′ to the start of translation of ZmRAD51 inPHP8057 by PCR. The PCR-modified ZmRAD51A and the ZmRAD51B genes fromPHP8057 and PHP8058 then were inserted as BamHI/Hpal fragmentsdownstream of the 2.0 kb PstI fragment of the maize ubiquitin promoterand upstream of the potato proteinase inhibitor II (PinII) terminator(bases 2 to 310 from An et al., Plant Cell, 1:115-122 (1989)) in a pUC19plasmid backbone to make PHP8060 (FIG. 1) and PHP8103 (FIG. 2).

Another set of constructs was made using either the ubiquitin promoteror the maize ZmDJ1 promoter (Baszczynski et al., Maydica, 42:189-201(1997)), but where the complete ZmRAD51A or ZmRAD51B genes were firstfused to the 3′ end of a green fluorescence protein (“GFP”; Chalfie etal., Science, 263:802-805 (1994)) gene that was previously synthesizedso as to encode maize-preferred codons (new gene designated “GFPm”) asdescribed in PCT Patent Application No. PCT/US97/07688. To construct theprotein fusions, the GFPm stop codon was removed and a BglII site wasgenerated by site-directed mutagenesis. This 0.8 kb BamHI/BglII fragmentof the GFPm coding sequence was inserted into the BamHI site 5′ to thestart of ZmRAD51 in PHP8060 and PHP8103 from above to create PHP8744(FIG. 3) and PHP8745, respectively. This process created fusions of GFPmto ZmRAD51A or ZmRAD51B joined by a 6 bp linker encoding isoleucine andhistidine (junctions shown below).

To create versions of these constructs utilizing the maize ZmdJ1promoter, the 1.8 kb BamHI/Hpal fragments containing GFPm/ZmRAD51A andGFPm/ZmRAD51B coding sequences from PHP8744 and PHP8745 were inserteddownstream of the 0.8 kb SacI/BglII ZmDJ1 promoter sequence and upstreamof the PinII terminator in a pBlueScript™ plasmid backbone to generatePHP8961 and PHP8962.

Example 6 Introduction of ZmRAD51 Gene Constructs into Maize Cells andDetection of Gene Expression

The various constructs described in Example 5 were introduced into cellsof the Black Mexican Sweet (“BMS”) maize line (Sheridan, J. Cell Biol.,67:396a (1975)) by particle gun bombardment using 1 ug of plasmid perparticle preparation at 6 shots per preparation. About 100 mg of BMScells per plate were shot. For experiments utilizing PHP8060 or PHP8103,another construct, PHP9053, which carried a fusion between a nuclearlocalization sequence (NLS), the GFPm gene as above and a maizeacetolactate synthase (ALS) gene (Fang et al., Plant Mol. Biol.,18:1185-1187 (1992)), all driven from the ubiquitin promoter was shotconcurrently. To create PHP9053, the nuclear localization signal fromsimian virus 40 (SV40) (Kalderon et al., Cell, 39:499-509 (1984)) wassynthesized as a BamHI/NcoI fragment and inserted at BamHI and AflIIIsites between the ubiquitin promoter and the start codon of GFPm. Inorder to enhance retention of the protein in the nucleus, the molecularweight of NLS/GFPm and hence the size of the protein was increased bymaking a carboxy terminal fusion with a large unrelated protein, in thiscase the maize ALS gene. The ALS coding sequence was inserted in frameat the GFPm 3′ BglII site and blunt-end ligated to the PinII terminator.Cells were viewed at 24-48 hours post-bombardment for GFP expressionusing a microscope equipped with epi-fluorescence and a FITC filter set.

In all cases, GFP expression was noted in the nucleus. At no time wasGFP fluorescence noted in the cytoplasm, either when GFP was part of afusion that included the NLS, or as a fusion only with ZmRAD51A orZmRAD51B. The data obtained with PHP8744, PHP8745, PHP8961 and PHP8962indicate that the expressed RAD51A or RAD51B proteins (in this case asfusions with GFP) localize to the nucleus in the absence of anexogenously added sequence known to facilitate nuclear localization(i.e., the SV40 NLS sequence). Comparable localization results wereobtained using two independent promoters (maize ubiquitin or the ZmDJ1promoters) indicating the information for nuclear localization islocated within the ZmRAD51 coding sequences. With constructs containingGFP alone, expression does not localize to the nucleus. The ZmRAD51nuclear localization sequence is located within in the 5′ end of thecoding region, preferably the first 40 amino acids of sequence SEQ IDNO: 3 or 7, more preferably the first 30 amino acids of SEQ ID NO: 3 or7, even more preferably the first 20 amino acids of SEQ ID NO: 3 or 7 ormost preferably the first 10 amino acids. The correspondingpolynucleotide sequence would be from nucleotide 53 to 113 of SEQ ID NO:1 or nucleotide 73 to 132 of SEQ ID NO:5 and fragments thereof.

As such, the methods and constructs disclosed provide a means ofintroducing maize RAD51 genes, or fusions of other genes with the maizeRAD51 genes into maize cells and maize nuclei, stably expressing thegene products under constitutive or inducible control and studying therole of these genes in plant cells.

Example 7 Expression of ZmRAD51 Genes in an E. coli Host Cell

E. coli expression vectors PHP9011 and PHP9012 were constructed byinsertion of BamHI/HpaI fragments containing the ZmRAD51A and ZmRAD51Bgenes from PHP8057 and PHP8058, respectively, into the BamHI and HindIIsites in a pET32c plasmid (Novagen, Inc., Madison, Wis.). The resultingplasmids consisted of the T7 promoter driving expression of a proteincontaining a 108 amino acid thioredoxin tag, a 6 amino acid histidinetag, a thrombin cleavage site, a 15 amino acid S tag (Novagen Inc.,Madison, Wis.), and an enterokinase cleavage site fused to the aminoterminus of one of the full length ZmRAD51 coding sequences.

The two constructs PHP9011 and PHP9012 were each transformed into theNovagen pET E. coli host strain AD494(DE3)pLysS, and used subsequentlyto express and purify ZmRAD51 A and ZmRAD51 B protein, respectively,according to the following procedure.

E. coli cells transformed with either the PHP9011 or the PHP9012expression vector were incubated in 2YT media (Life Technologies,Gaithersburg, Md.) containing, 100 ug/ml carbenicillin, and 34 ug/mlchloramphenicol. Cells were induced at approximately OD600=0.8 with 0.2mM IPTG (isopropyl-1-thio-β-D-galactoside) and incubated at roomtemperature for 3 hours.

The cells were then lysed in a lysis buffer containing 50 mM Tris-HCl(pH 8.0), 500 mM NaCl, 5 mM 2-mercaptoethanol, 1 mM PMSF(phenylmethylsulfonylfluoride), 0.1% Triton X-100, 10% Glycerol and 5 mMimidazole.

Purification of the cell lysate was carried out at 4° C. on a TALONmetal affinity column equilibrated with a solution containing 50 mMTris-HCl pH 8.0, 500 mM NaCl, 0.1% Triton X-100, 10% Glycerol and 5 mMimidazole (“equilibrium buffer”). Lysate was loaded onto theequilibrated TALON column. The loaded column was washed with theequilibrium buffer and then washed with a solution of 50 mM Tris-HCl (pH8.0), 10% Glycerol and 5mM imidazole. The washed column was then elutedwith a solution containing 50 mM Tris-HCl (pH 8.0), 10% Glycerol and 100mM Imidazole. 1 mM DTT and 1 mM EDTA were added to eluted protein, whichwas then stored at 4° C. The expressed fusion proteins were processedand purified as follows. The expressed fusion proteins were firstdialyzed to remove imidazole and the dialyzed fusion proteins weresite-specifically cleaved with enterokinase to remove the thioredoxintag. The cleavage products were purified on the Talon column to removethe cleaved tag fragment. Yields of protein were 3 mg/L for ZmRAD51A and1.5 mg/L for ZmRAD51B.

All publications and patent applications in this specification areindicative of the level of ordinary skill in the art to which thisinvention pertains. All publications and patent applications are hereinincorporated by reference to the same extent as if each individualpublication or patent application was specifically and individuallyindicated by reference.

The invention has been described with reference to various specific andpreferred embodiments and techniques. However, it should be understoodthat many variations and modifications may be made while remainingwithin the spirit and scope of the invention.

TABLE I FULL LENGTH cDNA AND CORRESPONDING AMINO ACID SEQUENCE FORZmRAD51A LOCUS ZMRAD51A (Sequence corresponding to cDNA insert inPHP7981) FEATURES peptide from 53 to 1072 ORIGIN Zea mays A632 line    1 GGCACGAGTTCGAACAGGGGCAGAGGTGAGACTTGAGAGAAGGAAGAAGGTCATGTCGTC                                                     M  S  S   61GGCGGCGCAGCAGCAGCAGAAAGCGGCGGCAGCGGAGCAGGAGGAGGTGGAGCACGGGCC A  A  Q  Q  Q  Q  K  A  A  A  A  E  Q  E  E  V  E  H  G  P  121ATTCCCCATCGAGCAGCTCCAGGCTTCTGGAATAGCTGCATTGGATGTGAAGAAGCTGAA F  P  I  E  Q  L  Q  A  S  G  I  A  A  L  D  V  K  K  L  K  181AGATTCTGGTCTCCACACTGTGGAGGCTGTGGCTTACACTCCAAGGAAAGATCTTCTGCA D  S  G  L  H  T  V  E  A  V  A  Y  T  P  R  K  D  L  L  Q  241GATCAAAGGGATAAGTGAAGCTAAAGCTGACAAGATAATTGAAGCAGCATCCAAGATAGT I  K  G  I  S  E  A  K  A  D  K  I  I  E  A  A  S  K  I  V  301TCCACTGGGATTTACAAGTGCCAGTCAACTTCATGCGCAGCGACTGGAGATTATTCAAGT P  L  G  F  T  S  A  S  Q  L  H  A  Q  R  L  E  I  I  Q  V  361TACAACTGGATCAAGAGAGCTTGATAAGATATTGGAGGGTGGGATAGAAACAGGATCTAT T  T  G  S  R  E  L  D  K  I  L  E  G  G  I  E  T  G  S  I  421CACTGAGATATATGGTGAGTTCCGCTCTGGAAAGACTCAGTTGTGTCACACCCCTTGTGT T  E  I  Y  G  E  F  R  S  G  K  T  Q  L  C  H  T  P  C  V  481TACATGTCAGCTTCCACTGGACCAGGGTGGTGGTGAAGGAAAGGCTCTATATATTGACGC T  C  Q  L  P  L  D  Q  G  G  G  E  G  K  A  L  Y  I  D  A  541AGAGGGTACATTCAGACCACAAAGGCTCTTGCAGATTGCTGACAGGTTTGGACTGAATGG E  G  T  F  R  P  Q  R  L  L  Q  I  A  D  R  F  G  L  N  G  601TGCTGATGTGTTAGAGAATGTGGCTTATGCCAGAGCTTATAATACGGATCATCAATCTAG A  D  V  L  E  N  V  A  Y  A  R  A  Y  N  T  D  H  Q  S  R  661ACTTCTGCTGGAAGCAGCTTCCATGATGATAGAGACCAGGTTTGCTCTTATGGTTGTAGA L  L  L  E  A  A  S  M  M  I  E  T  R  F  A  L  M  V  V  D  721CAGTGCCACAGCTCTGTACAGAACTGATTTCTCAGGAAGAGGGGAACTATCAGCGAGGCA S  A  T  A  L  Y  R  T  D  F  S  G  R  G  E  L  S  A  R  Q  781AATGCACATGGCTAAGTTCCTGAGGAGCCTTCAGAAGTTAGCTGATGAGTTTGGAGTAGC M  H  M  A  K  F  L  R  S  L  Q  K  L  A  D  E  F  G  V  A  841TGTGGTTATCACCAATCAAGTAGTGGCCCAAGTGGATGGATCTGCTATGTTTGCTGGACC V  V  I  T  N  Q  V  V  A  Q  V  D  G  S  A  M  F  A  G  P  901GCAGTTCAAGCCCATTGGTGGAAACATCATGGCTCATGCTTCAACCACAAGGCTTGCTCT Q  F  K  P  I  G  G  N  I  M  A  H  A  S  T  T  R  L  A  L  961TCGCAAGGGGCGAGGGGAGGAACGGATCTGTAAAGTAATAAGCTCTCCCTGCCTTGCTGA R  K  G  R  G  E  E  R  I  C  K  V  I  S  S  P  C  L  A  E 1021AGCCGAAGCAAGGTTTCAGTTAGCTTCTGAAGGTATTGCAGATGTTAAGGATTGAGACCA A  E  A  R  F  Q  L  A  S  E  G  I  A  D  V  K  D 1081TACCTGCTTTACAGGCATCTTCAGATCCATTGGTCTGCTATTTGCTTTGTCATTCCTTGG     G.TTAAC (HpaI) 1141GCCAACTTTCGTGTTGCCTCACCTTGATGTACAAAACGGTTTCGTTCACATATGTGAATG 1201CACGCCTGTGACTGATTTAGGCGTCCTGTTGTAAATAAAACGATGCCTGTTGCCCTGTTG 1261TGTGTTGCATGTAATCGACAACTCTACATATCACAATTATGATGTATTTTAGGTTTTATT 1321GTTCGCTTAGCACAGCCATTGCTGGATGTGCAATGTGGGATTATAGACAAGAATCCACAC 1381AACAACAATGGCCAATCCTGATAAAGTAGTTAGTGACTTGGGCAAATAGCATTGTGGTGA 1441TCTTTGAGTTCACTTGTGATAAGAACAGGGCTGGTGGCTGGTGGTGAAAACTAACTTGTG 1501ATCGGAACAGGTTTAATAGGGAAAACTAAGGATTCTATAAAAAAAAAAATAAAAAAAAAA 1561AAAAAAAA

TABLE II FULL LENGTH cDNA AND CORRESPONDING AMINO ACID SEQUENCE FORZmRAD51B LOCUS ZmRAD51B (Sequence corresponding to cDNA insert inPHP7983) FEATURES peptide from 53 to 1072 ORIGIN Zea mays A632 line    1GAATTCGGCACGAGATTTTTTGCCGCTTCGGAGGCACCTTCGAACAAAGCCCAAAAGCAG   61CCAGCGCACCGCATGTCCTCGTCTTCGGCGCACCAGAAGGCGTCGCCGCCGATAGAGGAG            M  S  S  S  S  A  H  Q  K  A  S  P  P  I  E  E     GGATCC(BamHI)  121GAAGCGACGGAGCACGGACCCTTCCCCATCGAACAGCTACAGGCATCTGGAATAGCTGCAE  A  T  E  H  G  P  F  P  I  E  Q  L  Q  A  S  G  I  A  A  181CTTGATGTGAAAAAACTCAAAGATGCTGGTCTCTGCACAGTGGAATCTGTAGCATACTCTL  D  V  K  K  L  K  D  A  G  L  C  T  V  E  S  V  A  Y  S  241CCAAGGAAAGACCTTTTGCAAATTAAAGGGATTAGTGAAGCCAAAGTCGACAAGATAATTP  R  K  D  L  L  Q  I  K  G  I  S  E  A  K  V  D  K  I  I  301GAAGCAGCTTCCAAGTTGGTTCCACTCGGATTTACTAGTGCTAGCCAACTTCATGCACAGE  A  A  S  K  L  V  P  L  G  F  T  S  A  S  Q  L  H  A  Q  361AGACTTGAGATCATCCAGCTTACAACTGGATCTAGAGAGCTTGATCAAATTTTGGACGGTR  L  E  I  I  Q  L  T  T  G  S  R  E  L  D  Q  I  L  D  G  421GGAATAGAAACAGGATCTATCACAGAGATGTATGGTGAATTTCGCTCCGGGAAGACTCAGG  I  E  T  G  S  I  T  E  M  Y  G  E  F  R  S  G  K  T  Q  481TTGTGCCACACTCTCTGTGTCACATGTCAGCTCCCATTGGACCAAGGTGGTGGTGAAGGAL  C  H  T  L  C  V  T  C  Q  L  P  L  D  Q  G  G  G  E  G  541AAGGCTTTGTATATTGATGCAGAGGGTACATTCAGGCCTCAAAGAATTCTCCAGATAGCAK  A  L  Y  I  D  A  E  G  T  F  R  P  Q  R  I  L  Q  I  A  601GACAGGTTTGGCTTGAATGGCGCTGATGTACTAGAGAATGTGGCTTATGCCAGAGCATATD  R  F  G  L  N  G  A  D  V  L  E  N  V  A  Y  A  R  A  Y  661AACACTGATCATCAATCAAGACTTTTGCTAGAAGCAGCCTCCATGATGGTAGAGACCAGGN  T  D  H  Q  S  R  L  L  L  E  A  A  S  M  M  V  E  T  R  721TTTGCTCTCATGGTTGTGGATAGTGCTACAGCCCTTTACAGAACTGATTTCTCTGGTAGAF  A  L  M  V  V  D  S  A  T  A  L  Y  R  T  D  F  S  G  R  781GGGGAGCTATCAGCAAGGCAGATGCATCTGGCGAAGTTTCTTAGGAGCCTTCAAAAGTTAG  E  L  S  A  R  Q  M  H  L  A  K  F  L  R  S  L  Q  K  L  841GCAGATGAGTTTGGAGTGGCAGTGGTAATCACGAACCAAGTAGTGGCTCAAGTGGATGGTA  D  E  F  G  V  A  V  V  I  T  N  Q  V  V  A  Q  V  D  G  901GCTGCAATGTTTGCTGGGCCACAGATCAAGCCCATTGGAGGGAACATCATGGCTCATGCTA  A  M  F  A  G  P  Q  I  K  P  I  G  G  N  I  M  A  H  A  961TCCACAACTAGGCTCTTTCTTCGCAAGGGAAGAGGGGAGGAGCGGATCTGCAAAGTAATCS  T  T  R  L  F  L  R  K  G  R  G  E  E  R  I  C  K  V  I 1021AGCTCTCCCTGCCTGGCTGAAGCTGAAGCAAGGTTTCAGATATCATCTGAGGGTGTCACTS  S  P  C  L  A  E  A  E  A  R  F  Q  I  S  S  E  G  V  T 1081GATGTCAAGGACTGAAAGCATCCTCATTTGCAGTCCACAGCATAACTTGCCAATTCAGACD  V  K  D                      GTTAAC (HpaI) 1141GAATCTCTGATCTGCTGCACTCGTGTCGGTCCCTTGTACAATCAAAATACCAGTACAGGC 1201TTCCAGAATGCGAATGCAAATCCGTTGGAGTGTGGCACTGTCATCCTGTTGTCTTTAGGT 1261ACCATCTAAAGTTGGCATTGTTGTAAAGTGGTAGAGCGCAAGGCTCTACTTTGTAGCCGT 1321GGATTCGAGCCCTATGGTGGGCGTTATTTAATTTTTTTGGCGAAAAAGCCTTTAATTGAG 1381TTGTTTAGGTGATATGAATAACTCTTTAGGTCATGGAGTTCGACTCCATGGGAGTTTAAG 1441CTGGGTTAAAAAAAATTATGGTCACGATCTTTTTCACATGGGCTACTGTAACATCTCGTC 1501TACTCCTGAACCGATGTTAAGCTTTTTAGGACTATAGATCATCTTCATATATCAACAAAA 1561AAAAAAAAAAAAAA

TABLE III RAD51A SEQUENCE-SPECIFIC PROBE FRAGMENT                                                      5′-CCA TACCTGCTTTACAGGCATCT TCAGATCCAT TGGTCTGCTA TTTGCTTTGT CATTCCTTGG GCCAACTTTCGTGTTGCCTC ACCTTGATGT ACAAAACGGT TTCGTTCACA TATGTGAATG CACGCCTGTGACTGATTTAG GCGTCCTGTT GTAAATAAAA CGATGCCTGT TGCCCTGTTG TGTGTTGCATGTAATCGACA ACTCTACATA TCACAATTAT GATGTATTTT AGGTTTTATT GTTCGCTTAGCACAGCCATT GCTGGATGTG CAATGTGGGA TTATAGACAA GAATCCACAC AACAACAATGGCCAATCCTG ATAAAGTAGT TAGTGACTTG GGCAAATAGC ATTGTGGTGA TCTTTGAGTTCACTTGTGAT AAGAACAGGG CTGGTGGCTG GTGGTGAAAA CTAACTTGTG ATCGGAACAGGTTTAATAGG GAAAACTAAG GATTCTATAA AAAAAAAAAT AAAAAAAAAA AAAAAAAACTCGAGGGGGGG CCCGGTACCC AATTCGCCCT ATAGTGAGTG AGTCGTATTA CAATTCACTGGCCGTCGTTT TACAACGTCG TGACTGGGA-3′

TABLE IV RAD51B SEQUENCE-SPECIFIC PROBE FRAGMENT                 5′-CATCCTCATTTG CAGTCCACAG CATAACTTGC CAATTCAGAC GAATCTCTGA TCTGCTGCACTCGTGTCGGT CCCTTGTACA ATCAAAATAC CAGTACAGGC TTCCAGAATG CGAATGCAAATCCGTTGGAG TGTGGCACTG TCATCCTGTT GTCTTTAGGT ACCATCTAAA GTTGGCATTGTTGTAAAGTG GTAGAGCGCA AGGCTCTACT TTGTAGCCGT GGATTCGAGC CCTATGGTGGGCGTTATTTA ATTTTTTTGG CGAAAAAGCC TTTAATTGAG TTGTTTAGGT GATATGAATAACTCTTTAGG TCATGGAGTT CGACTCCATG GGAGTTTAAG CTGGGTTAAA AAAAATTATGGTCACGATCT TTTTCACATG GGCTACTGTA ACATCTCGTC TACTCCTGAA CCGATGTTAAGCTTTTTAGG ACTATAGATC ATCTTCATAT ATCAACAAAA AAAAAAAAAA AAAACTCGAGGGGGGGCCCG GTACCCAATT CGCCCTATAG TGAGTGAGTC GTATTACAAT TCACTGGCCGTCGTTTTACA ACGTCGTGAC TGGGA-3′

TABLE V Polypeptide Sequence Similarities Between ZmRAD51A And ZmRAD51Band RAD51 Homologs From Other Higher Eukaryotes SOURCE OF RAD51 ZmRAD51AZmRAD51B SEQUENCE % similarity % identity % similarity % identityZmRAD51A 100.00 100.00 94.12 90.00 ZmRAD51B 94.12 90.00 100.00 100.00Tomato 92.65 86.76 94.12 89.12 Human 83.00 70.00 81.12 69.03 Mouse 82.7969.40 81.12 69.03 Chicken 81.60 68.84 80.53 68.73

TABLE VI MARKER DISTANCE ALIAS BIN P1057A 23.8 bn1 15.40 7.01/7.02P1158A 13.8 umc98 7.02 P9112A 12.2 npi112 7.02 P1054A 29.2 bn1 15.217.03 P1157A 1.5 umc110 7.03 P1147A 7.7 umc56 P5533A 15.3 jc629 P9263A13.1 npi263 7.04 P9240A 6.2 npi240 7.04 P8057A 2.1 rad51A P1173A 3.1umc125B 7.04 P1033A 20.9 bn18.32 7.04 P1036A 3.4 bn18.39 7.05 P5564A 3.6jc943 P554A 22.9 jc878 P3871A 4.5 umc151 7.05 P1059A 23.1 bn116.06 7.05P1037A 12.7 bn18.44 7.06 P1129A 10.1 umc35 7.06

TABLE VII MARKER DISTANCE ALIAS BIN P1035A 25.3 bn18.35 3.03 P1152A 0.0umc10 P8058A 5.4 rad51B P1044A 10.6 bn110.24 3.06 P1123A 7.1 umc60 3.06P1102A 3.2 umc82 P1017A 10.7 br16.16 3.07 P1185A 0.0 umc17 3.07 P9257A16.3 npi257 3.07 P1140a 19.2 umc63 3.08 P9457A npi457 3.09

24 1 1568 DNA Zea mays CDS (53)...(1072) 1 ggcacgagtt cgaacaggggcagaggtgag acttgagaga aggaagaagg tc atg tcg 58 Met Ser 1 tcg gcg gcg cagcag cag cag aaa gcg gcg gca gcg gag cag gag gag 106 Ser Ala Ala Gln GlnGln Gln Lys Ala Ala Ala Ala Glu Gln Glu Glu 5 10 15 gtg gag cac ggg ccattc ccc atc gag cag ctc cag gct tct gga ata 154 Val Glu His Gly Pro PhePro Ile Glu Gln Leu Gln Ala Ser Gly Ile 20 25 30 gct gca ttg gat gtg aagaag ctg aaa gat tct ggt ctc cac act gtg 202 Ala Ala Leu Asp Val Lys LysLeu Lys Asp Ser Gly Leu His Thr Val 35 40 45 50 gag gct gtg gct tac actcca agg aaa gat ctt ctg cag atc aaa ggg 250 Glu Ala Val Ala Tyr Thr ProArg Lys Asp Leu Leu Gln Ile Lys Gly 55 60 65 ata agt gaa gct aaa gct gacaag ata att gaa gca gca tcc aag ata 298 Ile Ser Glu Ala Lys Ala Asp LysIle Ile Glu Ala Ala Ser Lys Ile 70 75 80 gtt cca ctg gga ttt aca agt gccagt caa ctt cat gcg cag cga ctg 346 Val Pro Leu Gly Phe Thr Ser Ala SerGln Leu His Ala Gln Arg Leu 85 90 95 gag att att caa gtt aca act gga tcaaga gag ctt gat aag ata ttg 394 Glu Ile Ile Gln Val Thr Thr Gly Ser ArgGlu Leu Asp Lys Ile Leu 100 105 110 gag ggt ggg ata gaa aca gga tct atcact gag ata tat ggt gag ttc 442 Glu Gly Gly Ile Glu Thr Gly Ser Ile ThrGlu Ile Tyr Gly Glu Phe 115 120 125 130 cgc tct gga aag act cag ttg tgtcac acc cct tgt gtt aca tgt cag 490 Arg Ser Gly Lys Thr Gln Leu Cys HisThr Pro Cys Val Thr Cys Gln 135 140 145 ctt cca ctg gac cag ggt ggt ggtgaa gga aag gct cta tat att gac 538 Leu Pro Leu Asp Gln Gly Gly Gly GluGly Lys Ala Leu Tyr Ile Asp 150 155 160 gca gag ggt aca ttc aga cca caaagg ctc ttg cag att gct gac agg 586 Ala Glu Gly Thr Phe Arg Pro Gln ArgLeu Leu Gln Ile Ala Asp Arg 165 170 175 ttt gga ctg aat ggt gct gat gtgtta gag aat gtg gct tat gcc aga 634 Phe Gly Leu Asn Gly Ala Asp Val LeuGlu Asn Val Ala Tyr Ala Arg 180 185 190 gct tat aat acg gat cat caa tctaga ctt ctg ctg gaa gca gct tcc 682 Ala Tyr Asn Thr Asp His Gln Ser ArgLeu Leu Leu Glu Ala Ala Ser 195 200 205 210 atg atg ata gag acc agg tttgct ctt atg gtt gta gac agt gcc aca 730 Met Met Ile Glu Thr Arg Phe AlaLeu Met Val Val Asp Ser Ala Thr 215 220 225 gct ctg tac aga act gat ttctca gga aga ggg gaa cta tca gcg agg 778 Ala Leu Tyr Arg Thr Asp Phe SerGly Arg Gly Glu Leu Ser Ala Arg 230 235 240 caa atg cac atg gct aag ttcctg agg agc ctt cag aag tta gct gat 826 Gln Met His Met Ala Lys Phe LeuArg Ser Leu Gln Lys Leu Ala Asp 245 250 255 gag ttt gga gta gct gtg gttatc acc aat caa gta gtg gcc caa gtg 874 Glu Phe Gly Val Ala Val Val IleThr Asn Gln Val Val Ala Gln Val 260 265 270 gat gga tct gct atg ttt gctgga ccg cag ttc aag ccc att ggt gga 922 Asp Gly Ser Ala Met Phe Ala GlyPro Gln Phe Lys Pro Ile Gly Gly 275 280 285 290 aac atc atg gct cat gcttca acc aca agg ctt gct ctt cgc aag ggg 970 Asn Ile Met Ala His Ala SerThr Thr Arg Leu Ala Leu Arg Lys Gly 295 300 305 cga ggg gag gaa cgg atctgt aaa gta ata agc tct ccc tgc ctt gct 1018 Arg Gly Glu Glu Arg Ile CysLys Val Ile Ser Ser Pro Cys Leu Ala 310 315 320 gaa gcc gaa gca agg tttcag tta gct tct gaa ggt att gca gat gtt 1066 Glu Ala Glu Ala Arg Phe GlnLeu Ala Ser Glu Gly Ile Ala Asp Val 325 330 335 aag gat tgagaccatacctgctttac aggcatcttc agatccattg gtctgctatt 1122 Lys Asp 340 tgctttgtcattccttgggc caactttcgt gttgcctcac cttgatgtac aaaacggttt 1182 cgttcacatatgtgaatgca cgcctgtgac tgatttaggc gtcctgttgt aaataaaacg 1242 atgcctgttgccctgttgtg tgttgcatgt aatcgacaac tctacatatc acaattatga 1302 tgtattttaggttttattgt tcgcttagca cagccattgc tggatgtgca atgtgggatt 1362 atagacaagaatccacacaa caacaatggc caatcctgat aaagtagtta gtgacttggg 1422 caaatagcattgtggtgatc tttgagttca cttgtgataa gaacagggct ggtggctggt 1482 ggtgaaaactaacttgtgat cggaacaggt ttaataggga aaactaagga ttctataaaa 1542 aaaaaaataaaaaaaaaaaa aaaaaa 1568 2 1020 DNA Zea mays 2 atgtcgtcgg cggcgcagcagcagcagaaa gcggcggcag cggagcagga ggaggtggag 60 cacgggccat tccccatcgagcagctccag gcttctggaa tagctgcatt ggatgtgaag 120 aagctgaaag attctggtctccacactgtg gaggctgtgg cttacactcc aaggaaagat 180 cttctgcaga tcaaagggataagtgaagct aaagctgaca agataattga agcagcatcc 240 aagatagttc cactgggatttacaagtgcc agtcaacttc atgcgcagcg actggagatt 300 attcaagtta caactggatcaagagagctt gataagatat tggagggtgg gatagaaaca 360 ggatctatca ctgagatatatggtgagttc cgctctggaa agactcagtt gtgtcacacc 420 ccttgtgtta catgtcagcttccactggac cagggtggtg gtgaaggaaa ggctctatat 480 attgacgcag agggtacattcagaccacaa aggctcttgc agattgctga caggtttgga 540 ctgaatggtg ctgatgtgttagagaatgtg gcttatgcca gagcttataa tacggatcat 600 caatctagac ttctgctggaagcagcttcc atgatgatag agaccaggtt tgctcttatg 660 gttgtagaca gtgccacagctctgtacaga actgatttct caggaagagg ggaactatca 720 gcgaggcaaa tgcacatggctaagttcctg aggagccttc agaagttagc tgatgagttt 780 ggagtagctg tggttatcaccaatcaagta gtggcccaag tggatggatc tgctatgttt 840 gctggaccgc agttcaagcccattggtgga aacatcatgg ctcatgcttc aaccacaagg 900 cttgctcttc gcaaggggcgaggggaggaa cggatctgta aagtaataag ctctccctgc 960 cttgctgaag ccgaagcaaggtttcagtta gcttctgaag gtattgcaga tgttaaggat 1020 3 340 PRT Zea mays 3Met Ser Ser Ala Ala Gln Gln Gln Gln Lys Ala Ala Ala Ala Glu Gln 1 5 1015 Glu Glu Val Glu His Gly Pro Phe Pro Ile Glu Gln Leu Gln Ala Ser 20 2530 Gly Ile Ala Ala Leu Asp Val Lys Lys Leu Lys Asp Ser Gly Leu His 35 4045 Thr Val Glu Ala Val Ala Tyr Thr Pro Arg Lys Asp Leu Leu Gln Ile 50 5560 Lys Gly Ile Ser Glu Ala Lys Ala Asp Lys Ile Ile Glu Ala Ala Ser 65 7075 80 Lys Ile Val Pro Leu Gly Phe Thr Ser Ala Ser Gln Leu His Ala Gln 8590 95 Arg Leu Glu Ile Ile Gln Val Thr Thr Gly Ser Arg Glu Leu Asp Lys100 105 110 Ile Leu Glu Gly Gly Ile Glu Thr Gly Ser Ile Thr Glu Ile TyrGly 115 120 125 Glu Phe Arg Ser Gly Lys Thr Gln Leu Cys His Thr Pro CysVal Thr 130 135 140 Cys Gln Leu Pro Leu Asp Gln Gly Gly Gly Glu Gly LysAla Leu Tyr 145 150 155 160 Ile Asp Ala Glu Gly Thr Phe Arg Pro Gln ArgLeu Leu Gln Ile Ala 165 170 175 Asp Arg Phe Gly Leu Asn Gly Ala Asp ValLeu Glu Asn Val Ala Tyr 180 185 190 Ala Arg Ala Tyr Asn Thr Asp His GlnSer Arg Leu Leu Leu Glu Ala 195 200 205 Ala Ser Met Met Ile Glu Thr ArgPhe Ala Leu Met Val Val Asp Ser 210 215 220 Ala Thr Ala Leu Tyr Arg ThrAsp Phe Ser Gly Arg Gly Glu Leu Ser 225 230 235 240 Ala Arg Gln Met HisMet Ala Lys Phe Leu Arg Ser Leu Gln Lys Leu 245 250 255 Ala Asp Glu PheGly Val Ala Val Val Ile Thr Asn Gln Val Val Ala 260 265 270 Gln Val AspGly Ser Ala Met Phe Ala Gly Pro Gln Phe Lys Pro Ile 275 280 285 Gly GlyAsn Ile Met Ala His Ala Ser Thr Thr Arg Leu Ala Leu Arg 290 295 300 LysGly Arg Gly Glu Glu Arg Ile Cys Lys Val Ile Ser Ser Pro Cys 305 310 315320 Leu Ala Glu Ala Glu Ala Arg Phe Gln Leu Ala Ser Glu Gly Ile Ala 325330 335 Asp Val Lys Asp 340 4 461 DNA Zea mays 4 ccatacctgc tttacaggcatcttcagatc cattggtctg ctatttgctt tgtcattcct 60 tgggccaact ttcgtgttgcctcaccttga tgtacaaaac ggtttcgttc acatatgtga 120 atgcacgcct gtgactgatttaggcgtcct gttgtaaata aaacgatgcc tgttgccctg 180 ttgtgtgttg catgtaatcgacaactctac atatcacaat tatgatgtat tttaggtttt 240 attgttcgct tagcacagccattgctggat gtgcaatgtg ggattataga caagaatcca 300 cacaacaaca atggccaatcctgataaagt agttagtgac ttgggcaaat agcattgtgg 360 tgatctttga gttcacttgtgataagaaca gggctggtgg ctggtggtga aaactaactt 420 gtgatcggaa caggtttaatagggaaaact aaggattcta t 461 5 1574 DNA Zea mays CDS (73)...(1092) 5gaattcggca cgagattttt tgccgcttcg gaggcacctt cgaacaaagc ccaaaagcag 60ccagcgcacc gc atg tcc tcg tct tcg gcg cac cag aag gcg tcg ccg ccg 111Met Ser Ser Ser Ser Ala His Gln Lys Ala Ser Pro Pro 1 5 10 ata gag gaggaa gcg acg gag cac gga ccc ttc ccc atc gaa cag cta 159 Ile Glu Glu GluAla Thr Glu His Gly Pro Phe Pro Ile Glu Gln Leu 15 20 25 cag gca tct ggaata gct gca ctt gat gtg aaa aaa ctc aaa gat gct 207 Gln Ala Ser Gly IleAla Ala Leu Asp Val Lys Lys Leu Lys Asp Ala 30 35 40 45 ggt ctc tgc acagtg gaa tct gta gca tac tct cca agg aaa gac ctt 255 Gly Leu Cys Thr ValGlu Ser Val Ala Tyr Ser Pro Arg Lys Asp Leu 50 55 60 ttg caa att aaa gggatt agt gaa gcc aaa gtc gac aag ata att gaa 303 Leu Gln Ile Lys Gly IleSer Glu Ala Lys Val Asp Lys Ile Ile Glu 65 70 75 gca gct tcc aag ttg gttcca ctc gga ttt act agt gct agc caa ctt 351 Ala Ala Ser Lys Leu Val ProLeu Gly Phe Thr Ser Ala Ser Gln Leu 80 85 90 cat gca cag aga ctt gag atcatc cag ctt aca act gga tct aga gag 399 His Ala Gln Arg Leu Glu Ile IleGln Leu Thr Thr Gly Ser Arg Glu 95 100 105 ctt gat caa att ttg gac ggtgga ata gaa aca gga tct atc aca gag 447 Leu Asp Gln Ile Leu Asp Gly GlyIle Glu Thr Gly Ser Ile Thr Glu 110 115 120 125 atg tat ggt gaa ttt cgctcc ggg aag act cag ttg tgc cac act ctc 495 Met Tyr Gly Glu Phe Arg SerGly Lys Thr Gln Leu Cys His Thr Leu 130 135 140 tgt gtc aca tgt cag ctccca ttg gac caa ggt ggt ggt gaa gga aag 543 Cys Val Thr Cys Gln Leu ProLeu Asp Gln Gly Gly Gly Glu Gly Lys 145 150 155 gct ttg tat att gat gcagag ggt aca ttc agg cct caa aga att ctc 591 Ala Leu Tyr Ile Asp Ala GluGly Thr Phe Arg Pro Gln Arg Ile Leu 160 165 170 cag ata gca gac agg tttggc ttg aat ggc gct gat gta cta gag aat 639 Gln Ile Ala Asp Arg Phe GlyLeu Asn Gly Ala Asp Val Leu Glu Asn 175 180 185 gtg gct tat gcc aga gcatat aac act gat cat caa tca aga ctt ttg 687 Val Ala Tyr Ala Arg Ala TyrAsn Thr Asp His Gln Ser Arg Leu Leu 190 195 200 205 cta gaa gca gcc tccatg atg gta gag acc agg ttt gct ctc atg gtt 735 Leu Glu Ala Ala Ser MetMet Val Glu Thr Arg Phe Ala Leu Met Val 210 215 220 gtg gat agt gct acagcc ctt tac aga act gat ttc tct ggt aga ggg 783 Val Asp Ser Ala Thr AlaLeu Tyr Arg Thr Asp Phe Ser Gly Arg Gly 225 230 235 gag cta tca gca aggcag atg cat ctg gcg aag ttt ctt agg agc ctt 831 Glu Leu Ser Ala Arg GlnMet His Leu Ala Lys Phe Leu Arg Ser Leu 240 245 250 caa aag tta gca gatgag ttt gga gtg gca gtg gta atc acg aac caa 879 Gln Lys Leu Ala Asp GluPhe Gly Val Ala Val Val Ile Thr Asn Gln 255 260 265 gta gtg gct caa gtggat ggt gct gca atg ttt gct ggg cca cag atc 927 Val Val Ala Gln Val AspGly Ala Ala Met Phe Ala Gly Pro Gln Ile 270 275 280 285 aag ccc att ggaggg aac atc atg gct cat gct tcc aca act agg ctc 975 Lys Pro Ile Gly GlyAsn Ile Met Ala His Ala Ser Thr Thr Arg Leu 290 295 300 ttt ctt cgc aaggga aga ggg gag gag cgg atc tgc aaa gta atc agc 1023 Phe Leu Arg Lys GlyArg Gly Glu Glu Arg Ile Cys Lys Val Ile Ser 305 310 315 tct ccc tgc ctggct gaa gct gaa gca agg ttt cag ata tca tct gag 1071 Ser Pro Cys Leu AlaGlu Ala Glu Ala Arg Phe Gln Ile Ser Ser Glu 320 325 330 ggt gtc act gatgtc aag gac tgaaagcatc ctcatttgca gtccacagca 1122 Gly Val Thr Asp ValLys Asp 335 340 taacttgcca attcagacga atctctgatc tgctgcactc gtgtcggtcccttgtacaat 1182 caaaatacca gtacaggctt ccagaatgcg aatgcaaatc cgttggagtgtggcactgtc 1242 atcctgttgt ctttaggtac catctaaagt tggcattgtt gtaaagtggtagagcgcaag 1302 gctctacttt gtagccgtgg attcgagccc tatggtgggc gttatttaatttttttggcg 1362 aaaaagcctt taattgagtt gtttaggtga tatgaataac tctttaggtcatggagttcg 1422 actccatggg agtttaagct gggttaaaaa aaattatggt cacgatctttttcacatggg 1482 ctactgtaac atctcgtcta ctcctgaacc gatgttaagc tttttaggactatagatcat 1542 cttcatatat caacaaaaaa aaaaaaaaaa aa 1574 6 1020 DNA Zeamays 6 atgtcctcgt cttcggcgca ccagaaggcg tcgccgccga tagaggagga agcgacggag60 cacggaccct tccccatcga acagctacag gcatctggaa tagctgcact tgatgtgaaa 120aaactcaaag atgctggtct ctgcacagtg gaatctgtag catactctcc aaggaaagac 180cttttgcaaa ttaaagggat tagtgaagcc aaagtcgaca agataattga agcagcttcc 240aagttggttc cactcggatt tactagtgct agccaacttc atgcacagag acttgagatc 300atccagctta caactggatc tagagagctt gatcaaattt tggacggtgg aatagaaaca 360ggatctatca cagagatgta tggtgaattt cgctccggga agactcagtt gtgccacact 420ctctgtgtca catgtcagct cccattggac caaggtggtg gtgaaggaaa ggctttgtat 480attgatgcag agggtacatt caggcctcaa agaattctcc agatagcaga caggtttggc 540ttgaatggcg ctgatgtact agagaatgtg gcttatgcca gagcatataa cactgatcat 600caatcaagac ttttgctaga agcagcctcc atgatggtag agaccaggtt tgctctcatg 660gttgtggata gtgctacagc cctttacaga actgatttct ctggtagagg ggagctatca 720gcaaggcaga tgcatctggc gaagtttctt aggagccttc aaaagttagc agatgagttt 780ggagtggcag tggtaatcac gaaccaagta gtggctcaag tggatggtgc tgcaatgttt 840gctgggccac agatcaagcc cattggaggg aacatcatgg ctcatgcttc cacaactagg 900ctctttcttc gcaagggaag aggggaggag cggatctgca aagtaatcag ctctccctgc 960ctggctgaag ctgaagcaag gtttcagata tcatctgagg gtgtcactga tgtcaaggac 1020 7340 PRT Zea mays 7 Met Ser Ser Ser Ser Ala His Gln Lys Ala Ser Pro ProIle Glu Glu 1 5 10 15 Glu Ala Thr Glu His Gly Pro Phe Pro Ile Glu GlnLeu Gln Ala Ser 20 25 30 Gly Ile Ala Ala Leu Asp Val Lys Lys Leu Lys AspAla Gly Leu Cys 35 40 45 Thr Val Glu Ser Val Ala Tyr Ser Pro Arg Lys AspLeu Leu Gln Ile 50 55 60 Lys Gly Ile Ser Glu Ala Lys Val Asp Lys Ile IleGlu Ala Ala Ser 65 70 75 80 Lys Leu Val Pro Leu Gly Phe Thr Ser Ala SerGln Leu His Ala Gln 85 90 95 Arg Leu Glu Ile Ile Gln Leu Thr Thr Gly SerArg Glu Leu Asp Gln 100 105 110 Ile Leu Asp Gly Gly Ile Glu Thr Gly SerIle Thr Glu Met Tyr Gly 115 120 125 Glu Phe Arg Ser Gly Lys Thr Gln LeuCys His Thr Leu Cys Val Thr 130 135 140 Cys Gln Leu Pro Leu Asp Gln GlyGly Gly Glu Gly Lys Ala Leu Tyr 145 150 155 160 Ile Asp Ala Glu Gly ThrPhe Arg Pro Gln Arg Ile Leu Gln Ile Ala 165 170 175 Asp Arg Phe Gly LeuAsn Gly Ala Asp Val Leu Glu Asn Val Ala Tyr 180 185 190 Ala Arg Ala TyrAsn Thr Asp His Gln Ser Arg Leu Leu Leu Glu Ala 195 200 205 Ala Ser MetMet Val Glu Thr Arg Phe Ala Leu Met Val Val Asp Ser 210 215 220 Ala ThrAla Leu Tyr Arg Thr Asp Phe Ser Gly Arg Gly Glu Leu Ser 225 230 235 240Ala Arg Gln Met His Leu Ala Lys Phe Leu Arg Ser Leu Gln Lys Leu 245 250255 Ala Asp Glu Phe Gly Val Ala Val Val Ile Thr Asn Gln Val Val Ala 260265 270 Gln Val Asp Gly Ala Ala Met Phe Ala Gly Pro Gln Ile Lys Pro Ile275 280 285 Gly Gly Asn Ile Met Ala His Ala Ser Thr Thr Arg Leu Phe LeuArg 290 295 300 Lys Gly Arg Gly Glu Glu Arg Ile Cys Lys Val Ile Ser SerPro Cys 305 310 315 320 Leu Ala Glu Ala Glu Ala Arg Phe Gln Ile Ser SerGlu Gly Val Thr 325 330 335 Asp Val Lys Asp 340 8 458 DNA Zea mays 8catcctcatt tgcagtccac agcataactt gccaattcag acgaatctct gatctgctgc 60actcgtgtcg gtcccttgta caatcaaaat accagtacag gcttccagaa tgcgaatgca 120aatccgttgg agtgtggcac tgtcatcctg ttgtctttag gtaccatcta aagttggcat 180tgttgtaaag tggtagagcg caaggctcta ctttgtagcc gtggattcga gccctatggt 240gggcgttatt taattttttt ggcgaaaaag cctttaattg agttgtttag gtgatatgaa 300taactcttta ggtcatggag ttcgactcca tgggagttta agctgggtta aaaaaaatta 360tggtcacgat ctttttcaca tgggctactg taacatctcg tctactcctg aaccgatgtt 420aagcttttta ggactataga tcatcttcat atatcaac 458 9 582 DNA Zea mays 9ccatacctgc tttacaggca tcttcagatc cattggtctg ctatttgctt tgtcattcct 60tgggccaact ttcgtgttgc ctcaccttga tgtacaaaac ggtttcgttc acatatgtga 120atgcacgcct gtgactgatt taggcgtcct gttgtaaata aaacgatgcc tgttgccctg 180ttgtgtgttg catgtaatcg acaactctac atatcacaat tatgatgtat tttaggtttt 240attgttcgct tagcacagcc attgctggat gtgcaatgtg ggattataga caagaatcca 300cacaacaaca atggccaatc ctgataaagt agttagtgac ttgggcaaat agcattgtgg 360tgatctttga gttcacttgt gataagaaca gggctggtgg ctggtggtga aaactaactt 420gtgatcggaa caggtttaat agggaaaact aaggattcta taaaaaaaaa aataaaaaaa 480aaaaaaaaaa actcgagggg gggcccggta cccaattcgc cctatagtga gtgagtcgta 540ttacaattca ctggccgtcg ttttacaacg tcgtgactgg ga 582 10 567 DNA Zea mays10 catcctcatt tgcagtccac agcataactt gccaattcag acgaatctct gatctgctgc 60actcgtgtcg gtcccttgta caatcaaaat accagtacag gcttccagaa tgcgaatgca 120aatccgttgg agtgtggcac tgtcatcctg ttgtctttag gtaccatcta aagttggcat 180tgttgtaaag tggtagagcg caaggctcta ctttgtagcc gtggattcga gccctatggt 240gggcgttatt taattttttt ggcgaaaaag cctttaattg agttgtttag gtgatatgaa 300taactcttta ggtcatggag ttcgactcca tgggagttta agctgggtta aaaaaaatta 360tggtcacgat ctttttcaca tgggctactg taacatctcg tctactcctg aaccgatgtt 420aagcttttta ggactataga tcatcttcat atatcaacaa aaaaaaaaaa aaaaaactcg 480agggggggcc cggtacccaa ttcgccctat agtgagtgag tcgtattaca attcactggc 540cgtcgtttta caacgtcgtg actggga 567 11 360 DNA Zea mays 11 acattcagaccacaaaggct cttgcagatt gctgacaggt ttggactgaa tggtgctgat 60 gtgttagagaatgtggctta tgccagagct tataatacgg atcatcaatc tagacttctg 120 ctggaagcagcttccatgat gatagagacc aggtttactc ttatggttgt agacagtgcc 180 acagctctgtacagaactga tttctcagga agaggggaac tatcagcgag gcaaatgcac 240 atggctaagttcctgaggag ccttcagaag ttagctgatg agtttggagt agctgtggtt 300 atcaccaatcaagtagtggc ccaagtggat ggatctacta tgtttgctgg gccgcagttc 360 12 38 DNA Zeamays 12 tatagaattc cacaaaggct cttgcagatt gctgacag 38 13 37 DNA Zea mays13 atactcgagg cccagcaaac atagtagatc catccac 37 14 24 DNA Zea mays 14tcccagtcac gacgttgtaa aacg 24 15 36 DNA Zea mays 15 agcggataacaatttcacac aggaaacagc tatgac 36 16 51 DNA Zea mays 16 gtattgcagatgttaaggat tgagaccata cctggttaac aggcatctca g 51 17 27 DNA Zea mays 17gcagccaggg atccacatgt cctcgtc 27 18 52 DNA Zea mays 18 ctgatgtcaaggactgaaag catcctcatt tgcagttaac agcataactt gc 52 19 22 DNA Zea mays 19ccatacctgc tttacaggca tc 22 20 22 DNA Zea mays 20 catcctcatt tgsagtccacag 22 21 39 DNA Artificial Sequence GFPm to ZmRAD51A fusion, joined by a6 bp linker encoding isoleucine and histidine 21 gacgagctct acaagatccacatgtcgtcg gcggcgcag 39 22 12 PRT Artificial Sequence protein sequencefor GFPm to ZmRAD51A fusion, including the isoleucine and histidinelinker 22 Asp Glu Leu Tyr Lys Ile His Met Ser Ser Ala Ala 1 5 10 23 39DNA Artificial Sequence GFPm to ZmRAD51B fusion, joined by a 6 bp linkerencoding isoleucine and histidine 23 gacgagctct acaagatcca catgtcctcgtcttcggcg 39 24 13 PRT Artificial Sequence protein sequence for GFPm toZmRAD51B fusion, inlcuding isoleucine and histidine linker 24 Asp GluLeu Tyr Lys Ile His Met Ser Ser Ser Ser Ala 1 5 10

What is claimed is:
 1. An isolated polynucleotide comprising a memberselected from the group consisting of: a) a polynucleotide encoding apolypeptide selected from the group consisting of SEQ ID NO: 3 and SEQID NO: 7; b) a polynucleotide having at least 90% identity to apolynucleotide of (a); c) a polynucleotide which will hybridize understringent hybridization conditions to said polynucleotide of (a) or (b);and d) a polynucleotide comprising at least 30 contiguous nucleotidesfrom a polynucleotide of (a), (b) or (c); wherein the polynucleotide of(a), (b) or (c) encodes a polypeptide with recombinase activity.
 2. Theisolated polynucleotide of claim 1, wherein said polynucleotide has asequence selected from the group consisting of SEQ ID NO: 2 and SEQ IDNO:
 6. 3. An expression cassette comprising a polynucleotide of claim 1operably linked to a promoter.
 4. The host cell transfected with anexpression cassette of claim
 3. 5. The host cell of claim 4, whereinsaid host cell is a bacterial cell.
 6. The host cell of claim 4, whereinsaid host cell is a sorghum or maize cell.
 7. A method of making maizerecombinase comprising the steps of: a) transforming or transfecting ahost cell with the expression cassette of claim 3; and b) purifying therecombinase from the host cell.
 8. The method of claim 7, wherein thehost cell is selected from the group consisting of a bacterial cell, aplant cell, a mammalian cell and a yeast cell.
 9. A method of modulatingZmRAD 51 activity in a plant, comprising: (a) introducing into a plantcell an expression cassette comprising an isolated polynucleotide ofclaim 1 operatively linked to a promoter; (b) culturing the plant cellunder plant cell growing conditions; (c) regenerating a plant whichpossesses the transformed genotype, and (d) inducing expression of saidpolynucleotide for a time sufficient to modulate ZmRAD51 activity insaid plant.
 10. A transgenic plant cell comprising an isolatedpolynucleotide of claim
 1. 11. A transgenic plant comprising an isolatedpolynucleotide of claim
 1. 12. A transgenic seed from the transgenicplant of claim
 11. 13. Primer pairs for isolating at least a part of aZea mays recombinase gene, selected from the group consisting of SEQ IDNOS: 12 and 13, SEQ ID NOS: 14 and 19, SEQ IDS NOS: 14 and 20, and SEQID NOS: 14 and 15, or complements thereof.
 14. An RFLP probe for a maizerecombinase gene comprising at least 30 nucleotides residues of SEQ IDNO: 4, SEQ ID NO: 8, SEQ ID NO: 9, SEQ ID NO: 10, or SEQ ID NO: 11.